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aAD7709

Information furnished by Analog Devices is believed to be accurate andreliable. However, no responsibility is assumed by Analog Devices for itsuse, nor for any infringements of patents or other rights of third parties thatmay result from its use. No license is granted by implication or otherwiseunder any patent or patent rights of Analog Devices. Trademarks andregistered trademarks are the property of their respective companies.

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REV. A

16-Bit �-� ADC withSwitchable Current Sources

FUNCTIONAL BLOCK DIAGRAM

VDDIEXC1

8IIEXC2

8IIOUT1

IOUT2

IEXC3I

OSCILLATORANDPLL

XTAL2REFIN1(–) REFIN2(–)REFIN1(+) REFIN2(+) XTAL1

DOUT

RESET

RDY

CS

SCLK

DINSERIALINTERFACE

ANDCONTROL

LOGICBUF PGAMUX

I/O PORT

VDD

16-BIT �-� ADC

AIN1AIN2

AIN3/P3AIN4/P4AINCOM

AD7709

VDD GND PWRGND P1/SW1 P2/SW2

I = 25�A

FEATURES

16-Bit �-� ADC

Programmable Gain Front End

Simultaneous 50 Hz and 60 Hz Rejection at 20 Hz

Update Rate

VREF Select™ Allows Absolute and Ratiometric

Measurement Capability

ISOURCE Select™

16-Bit No Missing Codes

13-Bit p-p Resolution @ 20 Hz, 20 mV Range

16-Bit p-p Resolution @ 20 Hz, 2.56 V Range

INTERFACE

3-Wire Serial

SPI®, QSPI™, MICROWIRE™, and DSP Compatible

Schmitt Trigger on SCLK

POWER

Specified for Single 3 V and 5 V Operation

Normal: 1.25 mA Typ @ 3 V

Power-Down: 7 �A (32.768 kHz Crystal Running)

ON-CHIP FUNCTIONS

Rail-to-Rail Input Buffer and PGA

Selectable Reference Inputs

3 Switchable, Ratioed Current Sources for

VBE Measurements

4-Bit Digital I/O Port

Low-Side Power Switches

APPLICATIONS

Sensor Measurement

Temperature Measurement

Pressure Measurements

Weigh Scales

Portable Instrumentation

4–20 mA Loops

GENERAL DESCRIPTIONThe AD7709 is a complete analog front end for low frequencymeasurement applications. It contains a 16-bit �-� ADC, selectablereference inputs, three switchable matched excitation currentsources, low-side power switches, and a digital I/O port. The16-bit channel with PGA accepts fully differential, unipolar,and bipolar input signal ranges from 1.024 � REFIN/128 to1.024 � REFIN. It can be configured as two fully differentialinput channels or four pseudo-differential input channels. Signalscan be converted directly from a transducer without the need forsignal conditioning.

The device operates from a 32.768 kHz crystal with an on-chipPLL generating the required internal operating frequency. Theoutput data rate from the part is software programmable. Thep-p resolution from the part varies with the programmed gainand output data rate.

The part operates from a single 3 V or 5 V supply. Whenoperating from 3 V supplies, the power dissipation for the partis 3.75 mW. The AD7709 is housed in a 24-lead TSSOP package.

http://www.analog.com

REV. A

AD7709

–2–

TABLE OF CONTENTS

FEATURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1GENERAL DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . . 1FUNCTIONAL BLOCK DIAGRAM . . . . . . . . . . . . . . . . . 1SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3TIMING CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . 6ABSOLUTE MAXIMUM RATINGS . . . . . . . . . . . . . . . . . 8ORDERING GUIDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8PIN CONFIGURATION . . . . . . . . . . . . . . . . . . . . . . . . . . . 8PIN FUNCTION DESCRIPTIONS . . . . . . . . . . . . . . . . . . 9TYPICAL PERFORMANCE CHARACTERISTICS . . . . 10ADC CIRCUIT INFORMATION . . . . . . . . . . . . . . . . . . . 11

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11S-D ADC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

NOISE PERFORMANCE . . . . . . . . . . . . . . . . . . . . . . . . . 13ON-CHIP REGISTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Communications Register . . . . . . . . . . . . . . . . . . . . . . . . 14Status Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Configuration Register . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Filter Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18ADC Data Result Register . . . . . . . . . . . . . . . . . . . . . . . . 18

CONFIGURING THE AD7709 . . . . . . . . . . . . . . . . . . . . . 19DIGITAL INTERFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

MICROCOMPUTER/MICROPROCESSORINTERFACING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20AD7709-to-68HC11 Interface . . . . . . . . . . . . . . . . . . . . . 21AD7709-to-8051 Interface . . . . . . . . . . . . . . . . . . . . . . . . 21AD7709-to-ADSP-2103/ADSP-2105 Interface . . . . . . . . 21

CIRCUIT DESCRIPTION . . . . . . . . . . . . . . . . . . . . . . . . 22Analog Input Channels . . . . . . . . . . . . . . . . . . . . . . . . . . 22Programmable Gain Amplifier . . . . . . . . . . . . . . . . . . . . . 23Bipolar/Unipolar Configuration . . . . . . . . . . . . . . . . . . . . 23Data Output Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Excitation Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Crystal Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Reference Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Reset Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Power-Down Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Grounding and Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Pressure Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Temperature Measurement . . . . . . . . . . . . . . . . . . . . . . . 263-Wire RTD Configurations . . . . . . . . . . . . . . . . . . . . . . 27Smart Transmitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

OUTLINE DIMENSIONS . . . . . . . . . . . . . . . . . . . . . . . . . 29Revision History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

REV. A –3–

AD7709

SPECIFICATIONS1 (VDD = 2.7 V to 3.6 V or 4.75 V to 5.25 V, REFIN(+) = 2.5 V; REFIN(–) = GND; GND = 0 V; XTAL1/XTAL2 =32.768 kHz Crystal; all specifications TMIN to TMAX, unless otherwise noted.)

Parameter AD7709A, AD7709B Unit Test Conditions

ADC CHANNEL SPECIFICATIONOutput Update Rate 5.4 Hz min 0.732 ms Increments

105 Hz max

ADC CHANNELNo Missing Codes2 16 Bits min 20 Hz Update RateResolution 13 Bits p-p ± 20 mV Range, 20 Hz Update Rate

16 Bits p-p ± 2.56 V Range, 20 Hz Update RateOutput Noise and Update Rates See Tables II to VIntegral Nonlinearity2 ± 30 ppm of FSR max Typically 2 ppm

FSR

REFIN

GAIN=

¥2 1 024.

Offset Error ± 3 mV typOffset Error Drift vs. Temperature ± 10 nV/∞C typFull-Scale Error3 ± 0.75 LSB typ B Grade, VDD = 4 V

± 0.2 % of FS typ A GradeGain Drift vs. Temperature ± 0.5 ppm/∞C typPower Supply Rejection (PSR) 85 dB typ Input Range = ± 2.56 V

100 dB typ on ± 20 mV Range

ANALOG INPUTSDifferential Input Voltage Ranges

± ¥1 024. REFINGAIN V nom REFIN = REFIN(+) – REFIN(–)

GAIN = 1 to 128ADC Range Matching ± 2 mV typ Input Voltage = 19 mV on All RangesAbsolute AIN1–AIN4 Voltage Limits2 GND + 100 mV V min

VDD – 100 mV V maxAIN1–AIN4 Analog Input Current

DC Input Current2 ± 1 nA maxDC Input Current Drift ± 5 pA /∞C typ

Absolute AINCOM Voltage Limits2 GND – 30 mV V minVDD + 30 mV V max

AINCOM Analog Input Current Pseudo-Differential Mode of OperationDC Input Current ± 125 nA/V typ Input Current Varies with Input RangeDC Input Current Drift ± 2 pA/V/∞C typ

Normal-Mode Rejection2, 4

@ 50 Hz 100 dB min 50 Hz ± 1 Hz, 16.65 Hz Update Rate, SF = 82@ 60 Hz 100 dB min 60 Hz ± 1 Hz, 20 Hz Update Rate, SF = 68

Common-Mode Rejection@ DC 100 dB typ Input Range = ± 2.56 V, AIN = 1 V

110 dB typ on ± 20 mV Range@ 50 Hz2 100 dB min 50 Hz ± 1 Hz, Range = ± 2.56 V, AIN = 1 V@ 60 Hz2 100 dB min 60 Hz ± 1 Hz, Range = ± 2.56 V, AIN = 1 V

REFERENCE INPUTS(REFIN1 and REFIN2)

REFIN Voltage 2.5 V nom REFIN = REFIN(+) – REFIN(–)REFIN Voltage Range2 1 V min

VDD V maxAbsolute REFIN Voltage Limits2 GND – 30 mV V min

VDD + 30 mV V maxAverage Reference Input Current 0.5 mA/V typAverage Reference Input Current Drift ± 0.01 nA/V/∞C typNormal-Mode Rejection2, 4

@ 50 Hz 100 dB min 50 Hz ± 1 Hz, SF = 82@ 60 Hz 100 dB min 60 Hz ± 1 Hz, SF = 68

Common-Mode Rejection@ DC 110 dB typ Input Range = ± 2.56 V, AIN = 1 V@ 50 Hz 110 dB typ 50 Hz ± 1 Hz, Range = 2.56 V, AIN = 1 V@ 60 Hz 110 dB typ 60 Hz ± 1 Hz, Range = 2.56 V, AIN = 1 V

See Notes on page 5.

REV. A–4–

AD7709


EXCITATION CURRENT SOURCES(IEXC1, IEXC2, and IEXC3)

Output CurrentIEXC1, IEXC2 200 mA nomIEXC3 25 mA nom

Initial Tolerance at 25∞C ± 10 % typDrift 200 ppm/∞C typInitial Current Matching at 25∞C ± 2.5 % max B Grade, No Load

(between IEXC1 and IEXC2) ± 2.5 % typ A Grade, No LoadDrift Matching

(between IEXC1 and IEXC2) 20 ppm/∞C typInitial Current Matching at 25∞C ± 5 % max B Grade, No Load

(between 8 � IEXC3 andIEXC1/IEXC2) ± 5 % typ A Grade, No Load

Drift Matching(between 8 � IEXC3 andIEXC1/IEXC2) 20 ppm/∞C typ

Line Regulation VDD = 5 V ± 5%IEXC1, IEXC2 1.25 mA/V typ A, B Grades

2.6 mA/V max B GradeIEXC3 1 mA/V max B Grade

1 mA/V typ A GradeLoad Regulation 300 nA/V typOutput Compliance VDD – 0.6 V max

GND –30 mV V min

LOW-SIDE POWER SWITCHES(SW1 and SW2)

RON 3 W typ VDD = 5 V, A and B Grade5 W max B Grade4.5 W typ VDD = 3 V, A and B Grade7 W max B Grade

Allowable Current2 20 mA max Continuous Current per Switch

LOGIC INPUTSAll Inputs Except SCLK and XTAL12

VINL, Input Low Voltage 0.8 V max VDD = 5 V0.4 V max VDD = 3 V

VINH, Input High Voltage 2.0 V min VDD = 3 V or 5 VSCLK Only (Schmitt-Triggered Input)2

VT(+) 1.4/2 V min/V max VDD = 5 VVT(–) 0.8/1.4 V min/V max VDD = 5 VVT(+) – VT(–) 0.3/0.85 V min/V max VDD = 5 VVT(+) 0.95/2 V min/V max VDD = 3 VVT(–) 0.4/1.1 V min/V max VDD = 3 VVT(+) – VT(–) 0.3/0.85 V min/V max VDD = 3 V

XTAL1 Only2

VINL, Input Low Voltage 0.8 V max VDD = 5 VVINH, Input High Voltage 3.5 V min VDD = 5 VVINL, Input Low Voltage 0.4 V max VDD = 3 VVINH, Input High Voltage 2.5 V min VDD = 3 V

Input Currents (except XTAL) ± 2 mA max VIN = VDD

–70 mA max VIN = GND, Typically –40 mA @ 5 V and–20 mA at 3 V; Weak Pull-Ups on theLogic Inputs

Input Capacitance 10 pF typ All Digital Inputs

(continued)SPECIFICATIONS

REV. A


LOGIC OUTPUTS (Excluding XTAL2)VOH, Output High Voltage2 VDD – 0.6 V min VDD = 3 V, ISOURCE = 100 mAVOL, Output Low Voltage2 0.4 V max VDD = 3 V, ISINK = 100 mAVOH, Output High Voltage2 4 V min VDD = 5 V, ISOURCE = 200 mAVOL, Output Low Voltage2 0.4 V max VDD = 5 V, ISINK = 1.6 mAFloating-State Leakage Current ± 10 mA maxFloating-State Output Capacitance ± 10 pF typData Output Coding Binary Unipolar Mode

Offset Binary Bipolar Mode

I/O PORTVINL, Input Low Voltage2 0.8 V max VDD = 5 V

0.4 V max VDD = 3 VVINH, Input High Voltage2 2.0 V min VDD = 3 V or 5 VInput Currents ± 2 mA max VIN = VDD

–70 mA max VIN = GND, Typically –40 mA @ VDD = 5 Vand –20 mA at VDD = 3 V; Weak Pull-Ups onthe Logic Inputs

Input Capacitance 10 pF typ All Digital InputsVOH, Output High Voltage2 VDD – 0.6 V min VDD = 3 V, ISOURCE = 100 mAVOL, Output Low Voltage2 0.4 V max VDD = 3 V, ISINK = 100 mAVOH, Output High Voltage2 4 V min VDD = 5 V, ISOURCE = 200 mAVOL, Output Low Voltage2 0.4 V max VDD = 5 V, ISINK = 1.6 mAFloating-State Output Leakage Current ± 10 mA maxFloating-State Output Capacitance ± 10 pF typ

START-UP TIMEFrom Power-On 300 ms typFrom Standby Mode 1 ms typ OSCPD = 0From Power-Down Mode 300 ms typ OSCPD = 1

POWER REQUIREMENTSPower Supply Voltage

VDD – GND 2.7/3.6 V min/max VDD = 3 V nom4.75/5.25 V min/max VDD = 5 V nom

Power Supply CurrentsIDD Current 1.5 mA max VDD = 3 V, 1.25 mA typ

1.75 mA max VDD = 5 V, 1.45 mA typIDD (Low Power Mode) 7 mA max B Grade, VDD = 3 V, Standby Mode

7 mA typ A Grade, VDD = 3 V, Standby Mode1.5 mA max B Grade, VDD = 3 V, Power-Down Mode1.5 mA typ A Grade, VDD = 3 V, Power-Down Mode26 mA max B Grade, VDD = 5 V, Standby Mode26 mA typ A Grade, VDD = 5 V, Standby Mode6.5 mA max B Grade, VDD = 5 V, Power-Down Mode6.5 mA typ A Grade, VDD = 5 V, Power-Down Mode

IDD for One Conversion Second 1075 mA typ VDD = 3 V, Standby Mode1345 mA typ VDD = 5 V, Standby Mode

NOTES1Temperature Range –40∞C to +85∞C.2Guaranteed by design and/or characterization data on production release.3Full-scale error applies to both positive and negative full scale.4Simultaneous 50 Hz and 60 Hz rejection is achieved using 19.79 Hz update rate. Normal mode rejection in this case is 60 dB min.5When the part is placed in power-down mode for a single conversion/second, at an update rate of 19.79 Hz, the current consumption is higher compared to when thepart is placed in standby mode as the crystal oscillator takes approximately 100 ms to begin clocking. The device will, therefore, use full current for the conversiontime and the 100 ms period required for the oscillator to begin clocking. However, if the conversion rate is lower, the current consumption will be reduced so that itis worthwhile to use the power-down rather than the standby mode.

Specifications subject to change without notice.

AD7709

–5–

REV. A

AD7709

–6–

TIMING CHARACTERISTICS1, 2

Limit at TMIN, TMAXParameter (A, B Version) Unit Conditions/Comments

t1 30.5176 ms typ Crystal Oscillator Periodt2 50 ns min RESET PulsewidthRead Operation

t3 0 ns min RDY to CS Setup Timet4 0 ns min CS Falling Edge to SCLK Active Edge Setup Time3

t54 0 ns min SCLK Active Edge to Data Valid Delay3

60 ns max VDD = 4.75 V to 5.25 V80 ns max VDD = 2.7 V to 3.6 V

t5A4, 5 0 ns min CS Falling Edge to Data Valid Delay

60 ns max VDD = 4.75 V to 5.25 V80 ns max VDD = 2.7 V to 3.6 V

t6 100 ns min SCLK High Pulsewidtht7 100 ns min SCLK Low Pulsewidtht8 0 ns min CS Rising Edge to SCLK Inactive Edge Hold Time3

t96 10 ns min Bus Relinquish Time after SCLK Inactive Edge3

80 ns maxt10 100 ns max SCLK Active Edge to RDY High3, 7

Write Operationt11 0 ns min CS Falling Edge to SCLK Active Edge Setup Time3

t12 30 ns min Data Valid to SCLK Edge Setup Timet13 25 ns min Data Valid to SCLK Edge Hold Timet14 100 ns min SCLK High Pulsewidtht15 100 ns min SCLK Low Pulsewidtht16 0 ns min CS Rising Edge to SCLK Edge Hold Time

NOTES1Sample tested during initial release to ensure compliance. All input signals are specified with tR = tF = 5 ns (10% to 90% of VDD) and timed from a voltage level of 1.6 V.2See Figures 2 and 3.3SCLK active edge is falling edge of SCLK.4These numbers are measured with the load circuit of Figure 1 and defined as the time required for the output to cross the VOL or VOH limits.5This specification comes into play only if CS goes low while SCLK is low. It is required primarily for interfacing to DSP machines.6These numbers are derived from the measured time taken by the data output to change 0.5 V when loaded with the circuit of Figure 1. The measured number is then extrapo-lated back to remove effects of charging or discharging the 50 pF capacitor. This means that the times quoted in the Timing Characteristics table are the true bus relinquishtimes of the part and as such are independent of external bus loading capacitances.

7RDY returns high after a read of the ADC. The same data can be read again, if required, while RDY is high, although care should be taken that subsequent reads do not occurclose to the next output update.

(VDD = 2.7 V to 3.6 V or VDD = 4.75 V to 5.25 V; GND = 0 V; XTAL = 32.768 kHz; Input Logic 0 = 0 V,Logic 1 = VDD unless otherwise noted.)

REV. A

AD7709

–7–

ISINK (1.6mA WITH VDD = 5V100�A WITH VDD = 3V)

1.6V

ISOURCE (200�A WITH VDD = 5V100�A WITH VDD = 3V)

TO OUTPUTPIN

50pF

Figure 1. Load Circuit for Timing Characterization

t12

t13

t14

t15

t11 t16

MSB LSB

CS

SCLK

DIN

Figure 2. Write Cycle Timing Diagram

t5

t5A

t4 t6

t3

t9

MSB LSB

CS

SCLK

t8

t10

t7

DOUT

RDY

Figure 3. Read Cycle Timing Diagram

REV. A

AD7709

–8–

WARNING!

ESD SENSITIVE DEVICE

PIN CONFIGURATION

TOP VIEW(Not to Scale)

1

AD7709

GND

VDD

XTAL2

XTAL1

RDY

DOUT

DIN

RESET

SCLK

CS

P1/SW1

PWRGND

IOUT1

IOUT2

REFIN1(+)

REFIN1(–)

AIN1

AIN2

AIN3/P3

AIN4/P4

AINCOM

P2/SW2

REFIN2(+)

REFIN2(–)

2

3

4

5

6

7

8

9

10

11

12

24

23

22

21

20

19

18

17

16

15

14

13

ABSOLUTE MAXIMUM RATINGS*(TA = 25∞C, unless otherwise noted.)VDD to GND . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to +7 V

PWRGND to AGND . . . . . . . . . . . . . . –20 mV to +20 mVAnalog Input Voltage to GND . . . . . –0.3 V to VDD + 0.3 VReference Input Voltage to GND . . . –0.3 V to VDD + 0.3 VTotal AIN/REFIN Current (Indefinite) . . . . . . . . . . 30 mA

Digital Input Voltage to GND . . . . . . . –0.3 V to VDD + 0.3 VDigital Output Voltage to GND . . . . –0.3 V to VDD + 0.3 VOperating Temperature Range . . . . . . . . . . –40∞C to +85∞C

Storage Temperature Range . . . . . . . . . . . . –65∞C to +150∞CJunction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . 150∞C

qJA Thermal Impedance . . . . . . . . . . . . . . . . . . . 97.9∞C/WqJC Thermal Impedance . . . . . . . . . . . . . . . . . . . . . 14∞C/W

Lead Temperature, SolderingVapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . . . . 215∞CInfrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . . . 220∞C

*Stresses above those listed under Absolute Maximum Ratings may cause perma-nent damage to the device. This is a stress rating only; functional operation of thedevice at these or any other conditions above those listed in the operationalsections of this specification is not implied. Exposure to absolute maximum ratingconditions for extended periods may affect device reliability.

ORDERING GUIDE

Temperature Package PackageModel Range Description Option

AD7709ARU –40∞C to +85∞C TSSOP RU-24AD7709BRU –40∞C to +85∞C TSSOP RU-24EVAL-AD7709EB Evaluation Board

CAUTIONESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readilyaccumulate on the human body and test equipment and can discharge without detection. Although theAD7709 features proprietary ESD protection circuitry, permanent damage may occur on devicessubjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommendedto avoid performance degradation or loss of functionality.

REV. A

AD7709

–9–

PIN FUNCTION DESCRIPTIONS

Pin No. Mnemonic Function

1 IOUT1 Output for Internal Excitation Current Source. Either current source IEXC1, IEXC2, IEXC3, or a combina-tion of the current sources, can be switched to this output.

2 IOUT2 Output for Internal Excitation Current Source. Either current source IEXC1, IEXC2, IEXC3, or a combina-tion of the current sources, can be switched to this output.

3 REFIN1(+) Positive Reference Input. REFIN1(+) can lie anywhere between VDD and GND + 1 V. The nominal refer-ence voltage (REFIN1(+) – REFIN1(–)) is 2.5 V, but the part is functional with a reference range from 1 V to VDD.

4 REFIN1(–) Negative Reference Input. This reference input can lie anywhere between GND and VDD – 1 V.5 AIN1 Analog Input. Programmable gain input that can be used as a pseudo-differential input when used with

AINCOM or as the positive input of a fully differential input pair when used with AIN2.6 AIN2 Analog Input. Programmable gain input that can be used as a pseudo-differential input when used with

AINCOM or as the negative input of a fully differential input pair when used with AIN1.7 AIN3/P3 Analog Input/Digital Port Bit. Programmable gain input that can be used as a pseudo-differential input when

used with AINCOM or as the positive input of a fully differential input pair when used with AIN4. This pincan also be programmed as a general-purpose digital input bit.

8 AIN4/P4 Analog Input/Digital Port Bit. Programmable gain input that can be used as a pseudo-differential input whenused with AINCOM or as the negative input of a fully-differential input pair when used with AIN3. This pincan also be programmed as a general-purpose digital input bit.

9 AINCOM All analog inputs are referenced to this input when configured in pseudo-differential input mode.10 REFIN2(+) Positive Reference Input. REFIN2(+) can lie anywhere between VDD and GND + 1 V. The nominal reference

voltage (REFIN2(+) – REFIN2(–)) is 2.5 V, but the part is functional with a reference range from 1 V to VDD.11 REFIN2(–) Negative Reference Input. This reference input can lie anywhere between GND and VDD – 1 V.12 P2/SW2 Dual-Purpose Pin. It can act as a general-purpose output (P2) bit or as a low-side power switch (SW2) to

PWRGND.13 PWRGND Ground Point for the Low-Side Power Switches SW2 and SW1. PWRGND must be tied to GND.14 P1/SW1 Dual-Purpose Pin. It can act as a general-purpose output (P1) bit or as a low-side power switch (SW1) to

PWRGND.15 RESET Digital Input Used to Reset the ADC to Its Power-On-Reset Status. This pin has a weak pull-up internally to VDD.16 SCLK Serial Clock Input for Data Transfers to and from the ADC. The SCLK has a Schmitt-triggered input making

the interface suitable for opto-isolated applications. The serial clock can be continuous with all data transmittedin a continuous train of pulses. Alternatively, it can be a noncontinuous clock with the information beingtransmitted to or from the AD7709 in smaller batches of data. A weak pull-up to VDD is provided on theSCLK input.

17 CS Chip Select Input. This is an active low logic input used to select the AD7709. CS can be used to select theAD7709 in systems with more than one device on the serial bus or as a frame synchronization signal in com-municating with the device. CS can be hardwired low allowing the AD7709 to operate in 3-wire mode withSCLK, DIN, and DOUT used to interface with the device. A weak pull-up to VDD is provided on the CS input.

18 RDY RDY is a Logic Low Status Output from the AD7709. RDY is low if the ADC has valid data in its dataregister. This output returns high on completion of a read operation from the data register. If data is notread, RDY will return high prior to the next update indicating to the user that a read operation shouldnot be initiated.

19 DOUT Serial Data Output Accessing the Output Shift Register of the AD7709. The output shift register can containdata from any of the on-chip data or control registers.

20 DIN Serial Data Input Accessing the Input Shift Register on the AD7709. Data in this shift register is transferred tothe control registers within the ADC, the selection bits of the communications register selecting whichcontrol register. A weak pull-up to VDD is provided on the DIN input.

21 GND Ground Reference Point for the AD770922 VDD Supply Voltage, 3 V or 5 V Nominal23 XTAL2 Output from the 32.768 kHz Crystal Oscillator Inverter24 XTAL1 Input to the 32.768 kHz Crystal Oscillator Inverter

REV. A

AD7709–Typical Performance Characteristics

–10–

200

3276732766 32768 3277032769

600

500

400

300

700

CODE

OC

CU

RR

EN

CE

32771

100

0

TPC 3. Noise Histogram

OSCILLATOR

VDD = 5VTA = 25�C

TIME BASE = 100ms/DIVTRACE 1 = TRACE 2 = 2V/DIV

VDD

TPC 4. Typical Oscillator Power-Up

32767

1000 200 400300

32771

32770

32769

32768

32772

READING NUMBER

CO

DE

RE

AD

500

32766

32765

32764600 700 800 900 1000

VDD = 5VINPUT RANGE = �20mVUPDATE RATE = 19.79Hz

VREF = 2.5VTA = 25 C

TPC 1. Typical Noise Plot on ±20 mV Input Range

2.5

01.0 3.02.52.01.5 3.5 5.04.54.0

2.0

1.5

1.0

0.5

3.0

VREF – V

RM

S N

OIS

E –

�V

�20mV RANGE

�2.56V RANGE

VDD = 5VVREF = 2.5VINPUT RANGE = �2.56VUPDATE RATE = 19.79HzTA = 25�C

TPC 2. RMS Noise vs. Reference Input

REV. A

AD7709

–11–

ADC CIRCUIT INFORMATIONOverviewThe AD7709 incorporates a �-� ADC channel with on-chip digitalfiltering intended for the measurement of wide dynamic range, lowfrequency signals such as those in weigh-scale, strain-gauge,pressure transducer, or temperature measurement applications.

�-� ADCThis channel can be programmed to have one of eight inputvoltage ranges from ± 20 mV to ±2.56 V. This channel can beconfigured as either two fully differential inputs (AIN1/AIN2and AIN3/AIN4) or four pseudo-differential input channels(AIN1/AINCOM, AIN2/AINCOM, AIN3/AINCOM, andAIN4/AINCOM). Buffering the input channel means that thepart can accommodate significant source impedances on theanalog input and that R, C filtering (for noise rejection or RFIreduction) can be placed on the analog inputs if required.

The ADC employs a �-� conversion technique to realize up to16 bits of no-missing-codes performance. The �-� modulatorconverts the sampled input signal into a digital pulse train whoseduty cycle contains the digital information. A Sinc3 programmablelow-pass filter is then employed to decimate the modulator outputdata stream to give a valid data conversion result at programmableoutput rates from 5.35 Hz (186.77 ms) to 105.03 Hz (9.52 ms).A chopping scheme is also employed to minimize ADC channeloffset errors. A block diagram of the ADC input channel is shownin Figure 4.

The sampling frequency of the modulator loop is many timeshigher than the bandwidth of the input signal. The integrator inthe modulator shapes the quantization noise (which results fromthe analog-to-digital conversion) so that the noise is pushedtoward one-half of the modulator frequency. The output of the�-� modulator feeds directly into the digital filter. The digitalfilter then band-limits the response to a frequency significantlylower than one-half of the modulator frequency. In this manner,the 1-bit output of the comparator is translated into a band-limited, low noise output from the AD7709 ADC. The AD7709filter is a low-pass, Sinc3, or (SIN(x)/x)3 filter whose primaryfunction is to remove the quantization noise introduced at themodulator. The cutoff frequency and decimated output datarate of the filter are programmable via the SF word loaded to thefilter register.

A chopping scheme is employed where the complete signal chainis chopped, resulting in excellent dc offset and offset drift speci-fications, and is extremely beneficial in applications where drift,noise rejection, and optimum EMI rejection are important fac-tors. With chopping, the ADC repeatedly reverses its inputs.The decimated digital output words from the Sinc3 filters there-fore have a positive offset and negative offset term included. As aresult, a final summing stage is included so that each output

word from the filter is summed and averaged with the previousfilter output to produce a new valid output result to be written tothe ADC data register.

The input chopping is incorporated into the input multiplexerwhile the output chopping is accomplished by an XOR gate atthe output of the modulator. The chopped modulator bit streamis applied to a Sinc3 filter. The programming of the Sinc3 deci-mation factor is restricted to an 8-bit register SF, the actualdecimation factor is the register value × 8. The decimated out-put rate from the Sinc3 filter (and the ADC conversion rate) willtherefore be:

f

SFfADC MOD= ×

×

×13

18

where:

fADC is the ADC update rate.

SF is the decimal equivalent of the word loaded to thefilter register.

fMOD is the modulator sampling rate of 32.768 kHz.

Programming the filter register determines the update rate for theADC. The chop rate of the channel is half the output data rate.

The frequency response of the filter H(f ) is as follows:

18

8

12

2

3

SFSF f / f

f / f

f / ff / f

MOD

MOD

OUT

OUT

××

× × ××

×

×× ×

×

sin ( )sin ( )

sin ( )sin ( )

ππ

ππ

where:

fMOD = 32,768 Hz.

SF = value programmed into Filter Register.

fOUT = fMOD /(SF � 8 � 3)

The following shows plots of the filter frequency response for theSF words shown in Table I. The overall frequency response is theproduct of a Sinc3 and a sinc response. There are Sinc3 notchesat integer multiples of 3 � fADC, and there are sinc notches at oddinteger multiples of fADC /2. The 3 dB frequency for all values of SFobeys the following equation:

f dB fADC3 0 24( ) = ×.

The signal chain is chopped as shown in Figure 4. The chopfrequency is:

f

fCHOP

ADC=

2

SINC3 FILTER

MUX BUF PGA�-�

MODXORANALOG

INPUTDIGITALOUTPUT

18 � SF

�3

( (8 � SF )3 � �12

AIN + VOSAIN – VOS

fCHOP fIN fMOD fCHOP fADC

)

Figure 4. ADC Channel Block Diagram

REV. A

AD7709

–12–

As shown in the block diagram, the Sinc3 filter outputs alternatelycontain +VOS and –VOS, where VOS is the respective channel offset.This offset is removed by performing a running average of 2, whichmeans that the settling time to any change in programming ofthe ADC will be twice the normal conversion time, while anasynchronous step change on the analog input will not be fullyreflected until the third subsequent output.

t

ftSETTLE

ADCADC=

ÊËÁ

ˆ¯̃

= ¥22

The allowable range for SF is 13 to 255, with a default of 69(45H). The corresponding conversion rates, conversion times,and settling times are shown in Table I. Note that the conver-sion time increases by 0.732 ms for each increment in SF.

FREQUENCY – Hz

0

–140

–2000 65050 100 150 200 250 300 350 400 450 500 550 600 700

AT

TE

NU

AT

ION

– d

B

–20

–120

–160

–180

–60

–100

–40

–80

SF = 13OUTPUT DATA RATE = 105HzINPUT BANDWIDTH = 25.2HzFIRST NOTCH = 52.5Hz50Hz REJECTION = –23.6dB, 50Hz � 1Hz REJECTION = –20.5dB60Hz REJECTION = –14.6dB, 60Hz � 1Hz REJECTION = –13.6dB

Figure 5. Filter Profile with SF = 13

FREQUENCY – Hz

0

–80

–1600 10010

AT

TE

NU

AT

ION

– d

B

20 30 40 50 60 70 80 90

–20

–40

–120

–140

–60

–100

SF = 82OUTPUT DATA RATE = 16.65HzINPUT BANDWIDTH = 4Hz50Hz REJECTION = –171dB, 50Hz � 1Hz REJECTION = –100dB60Hz REJECTION = –58dB, 60Hz � 1Hz REJECTION = –53dB


Table I. ADC Conversion and Settling Times for VariousSF Words

Data Update Rate Settling TimeSF Word fADC (Hz) tSETTLE (ms)

13 105.3 19.0469 (Default) 19.79 101.07255 5.35 373.54

Normal mode rejection is the major function of the digital filteron the AD7709. The normal mode 50 ± 1 Hz rejection with anSF word of 82 is typically –100 dB. The 60 ± 1 Hz rejection withSF = 68 is typically –100 dB. Simultaneous 50 Hz and 60 Hzrejection of better than 60 dB is achieved with an SF of 69.Choosing an SF word of 69 places notches at both 50 Hz and60 Hz. Figures 5 to 8 show the filter rejection for a selectionof SF words.

FREQUENCY – Hz

0

–80

–1600 10010

AT

TE

NU

AT

ION

– d

B

20 30 40 50 60 70 80 90

–20

–40

–120

–140

–60

–100

SF = 69OUTPUT DATA RATE = 19.8HzINPUT BANDWIDTH = 4.74HzFIRST NOTCH = 9.9Hz50Hz REJECTION = –66dB, 50Hz � 1Hz REJECTION = –60dB60Hz REJECTION = –117dB, 60Hz � 1Hz REJECTION = –94dB

Figure 7. Filter Profile with Default SF = 69 Giving FilterNotches at Both 50 Hz and 60 Hz

FREQUENCY – Hz

0

–80

–1600 10010

AT

TE

NU

AT

ION

– d

B

20 30 40 50 60 70 80 90

–20

–40

–120

–140

–60

–100

SF = 255OUTPUT DATA RATE = 5.35HzINPUT BANDWIDTH = 1.28Hz50Hz REJECTION = –93dB, 50Hz � 1Hz REJECTION = –93dB60Hz REJECTION = –74dB, 60Hz � 1Hz REJECTION = –68dB


REV. A

AD7709

–13–

NOISE PERFORMANCETables II and III show the output rms noise and output peak-to-peak resolution in bits (rounded to the nearest 0.5 LSB) for aselection of output update rates. The numbers are typical andgenerated at a differential input voltage of 0 V. The output updaterate is selected via the SF7–SF0 bits in the Filter Register. It isimportant to note that the peak-to-peak resolution figuresrepresent the resolution for which there will be no code flickerwithin a six-sigma limit. The output noise comes from two sources.The first is the electrical noise in the semiconductor devices(device noise) used in the implementation of the modulator.Second, when the analog input is converted into the digitaldomain, quantization noise is added. The device noise is at a low

DIN

ADC STATUS REGISTER(8 BITS)

CONFIGURATION REGISTER(24 BITS)

FILTER REGISTER(8 BITS)

ADC DATA REGISTER(16 BITS)

REGISTERSELECT

DECODER

DOUT DOUT

DOUT

DOUT

DOUT

DIN

DIN

WEN R/W STBY OSCPD 0 0 A1 A0

Figure 9. On-Chip Registers

level and is independent of frequency. The quantization noise startsat an even lower level but rises rapidly with increasing frequencyto become the dominant noise source. The numbers in the tablesare given for the bipolar input ranges. For the unipolar ranges,the rms noise numbers will be the same as the bipolar range, butthe peak-to-peak resolution is now based on half the signal range,which effectively means losing 1 bit of resolution.

ON-CHIP REGISTERSThe AD7709 is controlled and configured via a number of on-chipregisters, as shown in Figure 9 and described in more detail in thefollowing pages. In the following descriptions, set implies a Logic 1state and cleared implies a Logic 0 state, unless otherwise stated.

Table II. Typical Output RMS Noise vs. Input Range and Update Rate for the AD7709 (Output RMS Noise in �V)

SF Data Update Input RangeWord Rate (Hz) �20 mV �40 mV �80 mV �160 mV �320 mV �640 mV �1.28 V �2.56 V

13 105.3 1.50 1.50 1.60 1.75 3.50 4.50 6.70 11.7569 19.79 0.60 0.65 0.65 0.65 0.65 0.95 1.40 2.30255 5.35 0.35 0.35 0.37 0.37 0.37 0.51 0.82 1.25

Table III. Peak-to-Peak Resolution vs. Input Range and Update Rate for the AD7709 (Peak-to-Peak Resolution in Bits)

SF Data Update Input RangeWord Rate (Hz) �20 mV �40 mV �80 mV �160 mV �320 mV �640 mV �1.28 V �2.56 V

13 105.3 12 13 14 15 15 15.5 16 1669 19.79 13 14 15 16 16 16 16 16255 5.35 14 15 16 16 16 16 16 16

REV. A

AD7709

–14–

Communications Register (A1, A0 = 0, 0)The Communications Register is an 8-bit write-only register. All communications to the part must start with a write operation to theCommunications Register. The data written to the Communications Register determines whether the next operation is a read orwrite operation, and to which register this operation takes place. For read or write operations, once the subsequent read or writeoperation to the selected register is complete, the interface returns to where it expects a write operation to the CommunicationsRegister. This is the default state of the interface, and on power-up or after a RESET, the AD7709 is in this default state waiting fora write operation to the Communications Register. In situations where the interface sequence is lost, a write operation of at least 32serial clock cycles with DIN high, returns the AD7709 to this default state by resetting the part. Table IV outlines the bit designations forthe Communications Register. CR0 to CR7 indicate the bit location, CR denoting the bits are in the Communications Register. CR7denotes the first bit of the data stream.

7RC 6RC 5RC 4RC 3RC 2RC 1RC 0RC

NEW )0( /R W )0( )0(YBTS )0(DPCSO )0(0 )0(0 )0(1A )0(A

Table IV. Communications Register Bit Designations

Bit BitLocation Name Description

CR7 WEN Write Enable Bit.A 0 must be written to this bit so the write operation to the Communications Register actually takes place.If a 1 is written to this bit, the part will not clock on to subsequent bits in the register. It will stay at thisbit location until a 0 is written to this bit. Once a 0 is written to the WEN bit, the next seven bits will beloaded to the Communications Register.

CR6 R/W A 0 in this bit location indicates that the next operation will be a write to a specified register.A 1 in this position indicates that the next operation will be a read from the designated register.

CR5 STBY Standby Bit Location.A 1 in this location places the AD7709 in low power mode.A 0 in this location powers up the AD7709.

CR4 OSCPD Oscillator Power-Down Bit.If this bit is set, placing the AD7709 in standby mode will stop the crystal oscillator also, reducing thepower consumed by the part to a minimum. The oscillator will require 300 ms to begin oscillating whenthe ADC is taken out of power-down mode.If this bit is cleared, the oscillator is not stopped when the ADC is placed in power-down mode. Whenthe ADC is taken out of power-down mode, the oscillator does not require the 300 ms start-up time.

CR3–CR2 0 These bits must be programmed with a Logic 0 for correct operation.

CR1–CR0 A1–A0 Register Address Bits. These address bits are used to select which of the AD7709 registers are accessedduring this serial interface communication.

Table V. Register Selection Table

A1 A0 Register

0 0 Communications Register during a Write Operation0 0 Status Register during a Read Operation0 1 Configuration Register1 0 Filter Register1 1 ADC Data Register

REV. A

AD7709

–15–

Status Register (A1, A0 = 0, 0; Power-On-Reset = 00H)The ADC Status Register is an 8-bit read-only register. To access the ADC Status Register, the user must write to the Communica-tions Register, selecting the next operation to be a read and load bits A1–A0 with 0, 0. Table VI outlines the bit designations for theStatus Register. SR0 to SR7 indicate the bit location, SR denoting the bits are in the Status Register. SR7 denotes the first bit of thedata stream. The number in brackets indicates the power-on-reset default status of that bit.

Table VI. Status Register Bit Designations


SR7 RDY Ready Bit for ADC.Set when data is written to the ADC data register.The RDY bit is cleared automatically after the ADC data register has been read or a period of time beforethe data register is updated with a new conversion result.

SR6 0 This bit is automatically cleared.



SR3 ERR ADC Error Bit. This bit is set at the same time as the RDY bit.Set to indicate that the result written to the ADC data register has been clamped to all zeros or all ones.Error sources include Overrange, Underrange.Cleared by a write to the mode bits to initiate a conversion.


SR1 STBY Standby Bit Indication.When this bit is set, the AD7709 is in power-down mode.This bit is cleared when the ADC is powered up.

SR0 LOCK PLL Lock Status Bit.Set if the PLL has locked onto the 32.768 kHz crystal oscillator clock. If the user is worried about exactsampling frequencies, etc., the LOCK bit should be interrogated and the result discarded if the LOCKbit is 0.

7RS 6RS 5RS 4RS 3RS 2RS 1RS 0RS

)0(YDR )0(0 )0(0 )0(0 )0(RRE )0(0 )0(YBTS )0(KCOL

REV. A

AD7709

–16–

Table VII. Configuration Register Bit Designations


CONFIG23 PSW2 Power Switch 2 Control Bit.Set by user to enable power switch SW2/P2 to PWRGND.Cleared by user to enable use as a standard I/O pin. When the ADC is in standby mode, the power switchesare open.

CONFIG22 PSW1 Power Switch 1 Control Bit.Set by user to enable power switch SW1/P1 to PWRGND.Cleared by user to enable use as a standard I/O pin. When the ADC is in standby mode, the power switchesare open.

CONFIG21 I3EN1 IEXC3 Current Source Enable Bit





Configuration Register (A1, A0 = 0, 1; Power-On-Reset = 000007H)The Configuration Register is a 24-bit register from which data can either be read or to which data can be written. This register is used toselect the input channel and configure the input range, excitation current sources, and I/O port. Table VII outlines the bit designationsfor this register. CONFIG23 to CONFIG0 indicate the bit location, CONFIG denoting the bits are in the Configuration Register.CONFIG23 denotes the first bit of the data stream. The number in brackets indicates the power-on-reset default status of that bit. Awrite to the Configuration Register has immediate effect and does not reset the ADC. Therefore, if a current source is switchedwhile the ADC is converting, the user will have to wait for the full settling time of the sinc3 filter before obtaining a fully settled output.This equates to three outputs.

7GIFNOC 7GIFNOC 7GIFNOC 7GIFNOC 7GIFNOC 6GIFNOC 6GIFNOC 6GIFNOC 6GIFNOC 6GIFNOC 5GIFNOC 5GIFNOC 5GIFNOC 5GIFNOC 5GIFNOC 4GIFNOC 4GIFNOC 4GIFNOC 4GIFNOC 4GIFNOC 3GIFNOC 3GIFNOC 3GIFNOC 3GIFNOC 3GIFNOC 2GIFNOC 2GIFNOC 2GIFNOC 2GIFNOC 2GIFNOC 1GIFNOC 1GIFNOC 1GIFNOC 1GIFNOC 1GIFNOC 0GIFNOC 0GIFNOC 0GIFNOC 0GIFNOC 0GIFNOC

)0(LESFER )0(LESFER )0(LESFER )0(LESFER )0(LESFER )0(2HC )0(2HC )0(2HC )0(2HC )0(2HC )0(1HC )0(1HC )0(1HC )0(1HC )0(1HC )0(0HC )0(0HC )0(0HC )0(0HC )0(0HC )0(INU )1(2NR )1(1NR )1(0NR

32GIFNOC 32GIFNOC 32GIFNOC 32GIFNOC 32GIFNOC 22GIFNOC 22GIFNOC 22GIFNOC 22GIFNOC 22GIFNOC 12GIFNOC 12GIFNOC 12GIFNOC 12GIFNOC 12GIFNOC 02GIFNOC 02GIFNOC 02GIFNOC 02GIFNOC 02GIFNOC 91GIFNOC 91GIFNOC 91GIFNOC 91GIFNOC 91GIFNOC 81GIFNOC 81GIFNOC 81GIFNOC 81GIFNOC 81GIFNOC 71GIFNOC 71GIFNOC 71GIFNOC 71GIFNOC 71GIFNOC 61GIFNOC 61GIFNOC 61GIFNOC 61GIFNOC 61GIFNOC

)0(2WSP )0(2WSP )0(2WSP )0(2WSP )0(2WSP )0(1WSP )0(1WSP )0(1WSP )0(1WSP )0(1WSP )0(1NE3I )0(1NE3I )0(1NE3I )0(1NE3I )0(1NE3I )0(0NE3I )0(0NE3I )0(0NE3I )0(0NE3I )0(0NE3I )0(1NE2I )0(1NE2I )0(1NE2I )0(1NE2I )0(1NE2I )0(0NE2I )0(0NE2I )0(0NE2I )0(0NE2I )0(0NE2I )0(1NE1I )0(1NE1I )0(1NE1I )0(1NE1I )0(1NE1I )0(0NE1I )0(0NE1I )0(0NE1I )0(0NE1I )0(0NE1I

I2EN1 I2EN0 Function

0 0 IEXC2 Current Source OFF0 1 IEXC2 Current Source Routed to the IOUT1 Pin1 0 IEXC2 Current Source Routed to the IOUT2 Pin1 1 Reserved



51GIFNOC 41GIFNOC 31GIFNOC 21GIFNOC 11GIFNOC 01GIFNOC 9GIFNOC 8GIFNOC

)0(GID4P )0(GID3P )0(NE2P )0(NE1P )0(TAD4P )0(TAD3P )0(TAD2P )0(TAD1P

REV. A

AD7709

–17–



CONFIG15 P4DIG Digital Input Enable.Set by user to enable pin AIN4/P4 as a digital input. A weak pull-up resistor is activated in this state.Cleared by user to configure pin AIN4/P4 as an analog input.

CONFIG14 P3DIG Digital Input Enable.Set by user to enable pin AIN3/P3 as a digital input. A weak pull-up resistor is activated in this state.Cleared by user to configure pin AIN3/P3 as an analog input.

CONFIG13 P2EN SW2/P2 Digital Output Enable Bit.Set by user to enable P2 as a regular digital output pin.Cleared by user to three-state the P2 output. PSW2 takes precedence over P2EN.

CONFIG12 P1EN SW1/P1 Digital Output Enable Bit.Set by user to enable P1 as a regular digital output pin.Cleared by user to three-state the P1 output. PSW1 takes precedence over P1EN.

CONFIG11 P4DAT Digital Input Port Data Bit.P4DAT is read only and will return a zero if P4DIG equals zero.If P4 is enabled as a digital input, the readback value indicates the status of pin P4.

CONFIG10 P3DAT Digital Input Port Data Bit.P3DAT is read only and will return a zero if P3DIG equals zero.If P3 is enabled as a digital input, the readback value indicates the status of pin P3.

CONFIG9 P2DAT Digital Output Port Data Bit. P2 is a digital output only. When the port is active as an output (P2EN = 1),the value written to this data bit appears at the output port. Reading P2DAT will return the last valuewritten to the P2DAT bit.

CONFIG8 P1DAT Digital Output Port Data Bit. P1 is a digital output only. When the port is active as an output (P1EN = 1),the value written to this data bit appears at the output port. Reading P1DAT will return the last valuewritten to the P1DAT bit.

CONFIG7 REFSEL ADC Reference Input Select.Cleared by the user to select REFIN1(+) and REFIN1(–) as the ADC reference.Set by the user to select REFIN2(+) and REFIN2(–) as the ADC reference.

CONFIG6 CH2 ADC Input Channel Selection Bit. It is used in conjunction with CH1 and CH0 as shown below.



The Buffer column indicates if the analog inputs are buffered or unbuffered. This determines the common-mode input rangeon each input. If the input is unbuffered (AINCOM), the common-mode input includes ground.

Table VII. Configuration Register Bit Designations (continued)

CH2 CH1 CH0 Positive Input Negative Input Buffer

0 0 0 AIN1 AINCOM Positive Analog Input0 0 1 AIN2 AINCOM Positive Analog Input0 1 0 AIN3 AINCOM Positive Analog Input0 1 1 AIN4 AINCOM Positive Analog Input1 0 0 AIN1 AIN2 Positive and Negative Analog Inputs1 0 1 AIN3 AIN4 Positive and Negative Analog Inputs1 1 0 AINCOM AINCOM None1 1 1 AIN2 AIN2 Positive and Negative Analog Inputs



REV. A

AD7709

–18–

Table VIII. Filter Register Bit Designations

Table IX. Update Rate vs. SF WORD

SF (Dec) SF (Hex) fADC (Hz) tADC (ms)

13 0D 105.3 9.5269 45 19.79 50.34255 FF 5.35 186.77

Table VII. Configuration Register Bit Designations (continued)


CONFIG3 UNI Unipolar/Bipolar Operation Selection Bit.Set by the user to enable unipolar operation. In this mode, the device uses straight binary output codingi.e., 0 differential input will generate a result of 0000h and a full-scale differential input will generate acode of FFFFh.Cleared by the user to enable pseudo-bipolar operation. The device uses offset binary coding, i.e., a nega-tive full-scale differential input will result in a code of 0000h, a 0 differential input will generate a code of8000h, while a positive full-scale differential input will result in a code of FFFFh.

CONFIG2 RN2 This bit is used in conjunction with RN1 and RN0 to select the analog input range as shown below.



Filter Register (A1, A0 = 1, 0; Power-On-Reset = 45h)The Filter Register is an 8-bit register from which data can beread or to which data can be written. This register determinesthe amount of averaging performed by the sinc filter. Table VIIIoutlines the bit designations for the Filter Register. FR7 throughFR0 indicate the bit location, FR denoting the bits are in theFilter Register. FR7 denotes the first bit of the data stream. Thenumber in brackets indicates the power-on/reset default statusof that bit. The number in this register is used to set the decima-tion factor and thus the output update rate for the ADC. TheFilter Register cannot be written to by the user while the ADCis active. The update rate is calculated as follows:

f

SFfADC MOD= ¥

¥¥1

31

8where:

fADC is the ADC output update rate.fMOD is the Modulator Clock Frequency = 32.768 kHz.SF is the decimal value written to the SF Register.

The allowable range for SF is 13dec to 255dec. Examples of SFvalues and corresponding conversion rate (fADC) and time (tADC)are shown in Table IX. It should also be noted that the ADCinput channel is chopped to minimize offset errors. This meansthat the time for a single conversion or the time to the first con-version result is 2 � tADC.

ADC Data Result Register (A1, A0 = 1, 1; Power-On-Reset =0000h)The conversion result is stored in the ADC Data Register (DATA).This register is 16-bits wide. This is a read-only register. Oncompletion of a read from this register, the RDY bit in theStatus Register is cleared.

7RF 6RF 5RF 4RF 3RF 2RF 1RF 0RF

)0(7FS )1(6FS )0(5FS )0(4FS )0(3FS )1(2FS )0(1FS )1(0FS

RN2 RN1 RN0 Selected ADC Input Range (VREF = 2.5 V)

0 0 0 ± 20 mV0 0 1 ± 40 mV0 1 0 ± 80 mV0 1 1 ± 160 mV1 0 0 ± 320 mV1 0 1 ± 640 mV1 1 0 ± 1.28 V1 1 1 ± 2.56 V

REV. A

AD7709

–19–

CONFIGURING THE AD7709The four user-accessible registers on the AD7709 are accessed viathe serial interface. Communication with any of these registersis initiated by first writing to the Communications Register. TheAD7709 begins converting on power-up without the need towrite to the registers. The default conditions are used, i.e., theAD7709 operates at a 19.79 Hz update rate that offers 50 Hzand 60 Hz rejection.

Figure 10 outlines a flow diagram of the sequence used toconfigure all registers after a power-up or reset on the AD7709.The flowchart shows two methods of determining when it is validto read the data register. The first method is hardware polling ofthe RDY pin and the second method involves software interrogationof the RDY bit in the status register. The flowchart details all thenecessary programming steps required to initialize the ADC andread data from the ADC channel following a power-on or reset.The steps can be broken down as follows:

1. Configure and initialize the microcontroller or microproces-sor serial port.

2. Initialize the AD7709 by configuring the following registers:

a) Filter Register to configure the update rate for the channel.The AD7709 must be placed in standby mode before theFilter Register can be written to.

b)Configuration Register to select the input channel to beconverted, its input range, and reference. This register is alsoused to configure internal current sources, power switches,and I/O port.

Both of these operations consist of a write to the Communi-cations Register to specify the next operation as a write to aspecified register. Data is then written to this register. Wheneach sequence is complete, the ADC defaults to waiting foranother write to the Communications Register to specify thenext operation.

3. When configuration is complete, the user needs to determinewhen it is valid to read the data from the data register. This isaccomplished either by polling the RDY pin (hardware polling)or by interrogating the RDY bit in the STATUS register(software polling). Both are shown in Figure 10.

POLL RDY PIN

HARDWAREPOLLING

STARTSOFTWARE

POLLING

POWER-ON-RESET FOR AD7709

CONFIGURE AND INITIALIZE �C/�P SERIAL PORT

WRITE TO COMMUNICATIONS REGISTER SETTINGUP NEXT OPERATION TO BE A WRITE TO THEFILTER REGISTER (WRITE 22H TO REGISTER)

WRITE TO FILTER REGISTER CONFIRMINGTHE REQUIRED UPDATE RATE

WRITE TO COMMUNICATIONS REGISTER SETTINGUP NEXT OPERATION TO BE A WRITE TO THE

CONFIGURATION REGISTER(WRITE 01H TO REGISTER)

READ DATA FROM OUTPUT REGISTER

WRITE TO COMMUNICATIONS REGISTER SETTINGUP NEXT OPERATION TO BE A READ FROM THESTATUS REGISTER (WRITE 40H TO REGISTER)

WRITE TO COMMUNICATIONS REGISTER SETTINGUP NEXT OPERATION TO BE A READ FROM THE

DATA REGISTER (WRITE 43H TO REGISTER)

WRITE TO COMMUNICATIONS REGISTER SETTINGUP NEXT OPERATION TO BE A READ FROM THE

DATA REGISTER (WRITE 43H TO REGISTER)

READ 16-BIT DATA RESULT

READ 16-BIT DATA RESULT

READ STATUS REGISTER

ANOTHERREAD

RDYLOW?

ANOTHERREAD

CHANNELCHANGE

RDY = 1

END

END

SOFTWAREPOLLING

HARDWAREPOLLING

NO

YES

YES

YESNO

YES

NO

YES

NO

YESNO

WRITE TO CONFIGURATION REGISTER TO SELECTTHE INPUT CHANNEL, INPUT RANGE, AND

REFERENCE. CURRENT SOURCES AND I/O PORTCAN ALSO BE CONFIGURED

CHANNELCHANGE

Figure 10. Flowchart for Initializing and Reading Data from the AD7709

REV. A

AD7709

–20–

DIGITAL INTERFACEAs previously outlined, AD7709 programmable functions arecontrolled using a set of on-chip registers. Data is written tothese registers via the part’s serial interface and read access tothe on-chip registers is also provided by this interface. All com-munications to the part must start with a write operation to theCommunications Register. After power-on or reset, the deviceexpects a write to its Communications Register. The data writ-ten to this register determines whether the next operation to thepart is a read or a write operation and also determines to whichregister this read or write operation occurs. Therefore, writeaccess to any of the other registers on the part starts with a writeoperation to the Communications Register followed by a writeto the selected register. A read operation from any other registeron the part (including the output data register) starts with awrite operation to the Communications Register followed by aread operation from the selected register.

The AD7709 serial interface consists of five signals: CS, SCLK,DIN, DOUT, and RDY. The DIN line is used for transferringdata into the on-chip registers, while the DOUT line is used foraccessing data from the on-chip registers. SCLK is the serialclock input for the device, and all data transfers (either on DINor DOUT) take place with respect to this SCLK signal. TheRDY line is used as a status signal to indicate when data is readyto be read from the AD7709 data register. RDY goes low when anew data-word is available in the output register. It is reset highwhen a read operation from the data register is complete. It alsogoes high prior to the updating of the output register to indicatewhen not to read from the device to ensure that a data read is notattempted while the register is being updated. CS is used to selectthe device. It can be used to decode the AD7709 in systems wherea number of parts are connected to the serial bus.

Figures 2 and 3 show timing diagrams for interfacing to theAD7709 with CS used to decode the part. Figure 3 is for a readoperation from the AD7709 output shift register while Figure 2shows a write operation to the input shift register. It is possibleto read the same data twice from the output register even thoughthe RDY line returns high after the first read operation. Care mustbe taken, however, to ensure that the read operations have beencompleted before the next output update is about to take place.

The AD7709 serial interface can operate in 3-wire mode bytying the CS input low. In this case, the SCLK, DIN, andDOUT lines are used to communicate with the AD7709, andthe status of the RDY bit can be obtained by interrogating theStatus Register. This scheme is suitable for interfacing tomicrocontrollers. If CS is required as a decoding signal, it canbe generated from a port bit. For microcontroller interfaces, it isrecommended that the SCLK idles high between data transfers.

The AD7709 can also be operated with CS used as a framesynchronization signal. This scheme is suitable for DSP interfaces.In this case, the first bit (MSB) is effectively clocked out by CSsince CS would normally occur after the falling edge of SCLKin DSPs. The SCLK can continue to run between data transfersprovided the timing numbers are obeyed.

The serial interface can be reset by exercising the RESET inputon the part. It can also be reset by writing a series of 1s on theDIN input. If a Logic 1 is written to the AD7709 DIN line forat least 32 serial clock cycles, the serial interface is reset. Thisensures that in 3-wire systems, if the interface gets lost either viaa software error or by some glitch in the system, it can be resetback to a known state. This state returns the interface to wherethe AD7709 is expecting a write operation to its CommunicationsRegister. This operation resets the contents of all registers to theirpower-on reset values.

Some microprocessor or microcontroller serial interfaces have asingle serial data line. In this case, it is possible to connect theAD7709 DOUT and DIN lines together and connect them to thesingle data line of the processor. A 10 kW pull-up resistor shouldbe used on this single data line. In this case, if the interface getslost, because the read and write operations share the same line,the procedure to reset it back to a known state is somewhatdifferent than previously described. It requires a read operationof 24 serial clocks followed by a write operation where a Logic 1is written for at least 32 serial clock cycles to ensure that theserial interface is back into a known state.

MICROCOMPUTER/MICROPROCESSOR INTERFACINGThe AD7709 flexible serial interface allows for easy interface tomost microcomputers and microprocessors. The flowchart ofFigure 10 outlines the sequence that should be followed wheninterfacing a microcontroller or microprocessor to the AD7709.Figures 11, 12, and 13 show some typical interface circuits. Theserial interface on the AD7709 is capable of operating from justthree wires and is compatible with SPI interface protocols. The3-wire operation makes the part ideal for isolated systems whereminimizing the number of interface lines minimizes the numberof opto-isolators required in the system. The serial clock input isa Schmitt-triggered input to accommodate slow edges fromopto-couplers. The rise and fall times of other digital inputs tothe AD7709 should be no longer than 1 ms.

Some of the registers on the AD7709 are 8-bit registers, whichfacilitates easy interfacing to the 8-bit serial ports of microcon-trollers. The Data Register on the AD7709 is 16 bits and theConfiguration Register is 24 bits, but data transfers to theseregisters can consist of multiple 8-bit transfers to the serial portof the microcontroller. DSP processors and microprocessorsgenerally transfer 16 bits of data in a serial data operation. Someof these processors, such as the ADSP-2105, have the facility toprogram the amount of cycles in a serial transfer. This allows theuser to tailor the number of bits in any transfer to match theregister length of the required register in the AD7709.

Even though some of the registers on the AD7709 are only 8 bitsin length, communicating with two of these registers in successivewrite operations can be handled as a single 16-bit data transfer ifrequired. For example, if the Filter Register is to be updated, theprocessor must first write to the Communications Register (say-ing that the next operation is a write to the Filter Register), andthen write 8 bits to the Filter Register. If required, this can all bedone in a single 16-bit transfer because once the eight serialclocks of the write operation to the Communications Registerhave been completed, the part immediately sets itself up for awrite operation to the Filter Register.

REV. A

AD7709

–21–

AD7709-to-68HC11 InterfaceFigure 11 shows an interface between the AD7709 and the68HC11 microcontroller. The diagram shows the minimum(3-wire) interface with CS on the AD7709 hardwired low. In thisscheme, the RDY bit of the Status Register is monitored todetermine when the Data Register is updated. An alternativescheme, which increases the number of interface lines to four, is tomonitor the RDY output line from the AD7709. The monitoringof the RDY line can be done in two ways. First, RDY can beconnected to one of the 68HC11 port bits (such as PC0), whichis configured as an input. This port bit is then polled to determinethe status of RDY. The second scheme is to use an interruptdriven system, in which case the RDY output is connected tothe IRQ input of the 68HC11. For interfaces that requirecontrol of the CS input on the AD7709, one of the port bits of the68HC11 (such as PC1), which is configured as an output, canbe used to drive the CS input.

The 68HC11 is configured in the master mode with its CPOLbit set to a Logic 1 and its CPHA bit set to a Logic 1. When the68HC11 is configured like this, its SCLK line idles high betweendata transfers. The AD7709 is not capable of full-duplex opera-tion. If the AD7709 is configured for a write operation, no dataappears on the DOUT lines even when the SCLK input is active.Similarly, if the AD7709 is configured for a read operation, datapresented to the part on the DIN line is ignored even whenSCLK is active.

68HC11 VDD AD7709

SS

SCK

MISO

MOSI

RESET

DOUT

DIN

CS

SCLK

VDD

Figure 11. AD7709-to-68HC11 Interface

AD7709-to-8051 InterfaceAn interface circuit between the AD7709 and the 8XC51 microcon-troller is shown in Figure 12. The diagram shows the minimumnumber of interface connections with CS on the AD7709 hard-wired low. In the case of the 8XC51 interface, the minimumnumber of interconnects is just two. In this scheme, the RDYbit of the Status Register is monitored to determine when theData Register is updated. The alternative scheme, which increasesthe number of interface lines to three, is to monitor the RDY outputline from the AD7709. The monitoring of the RDY line can bedone in two ways. First, RDY can be connected to one of the8XC51 port bits (such as P1.0) which is configured as an input.This port bit is then polled to determine the status of RDY.

8XC51VDD AD7709

P3.0

P3.1

RESET

DIN

SCLK

CS

DOUT

VDD

10k�

Figure 12. AD7709-to-8XC51 Interface

The second scheme is to use an interrupt-driven system, in whichcase the RDY output is connected to the INT1 input of the8XC51. For interfaces that require control of the CS input onthe AD7709, one of the port bits of the 8XC51 (such as P1.1),which is configured as an output, can be used to drive the CSinput. The 8XC51 is configured in its Mode 0 serial interfacemode. Its serial interface contains a single data line. As a result,the DOUT and DIN pins of the AD7709 should be connectedtogether with a 10 kW pull-up resistor. The serial clock on the8XC51 idles high between data transfers. The 8XC51 outputsthe LSB first in a write operation, while the AD7709 expects theMSB first so the data to be transmitted has to be rearrangedbefore being written to the output serial register. Similarly, theAD7709 outputs the MSB first during a read operation whilethe 8XC51 expects the LSB first. Therefore, the data read intothe serial buffer needs to be rearranged before the correct dataword from the AD7709 is available in the accumulator.

ADSP-2103/ADSP-2105

VDD AD7709

RFS

SCLK

RESET

DOUT

DIN

SCLK

CSTFS

DR

DT

Figure 13. AD7709-to-ADSP-2103/ADSP-2105 Interface

AD7709-to-ADSP-2103/ADSP-2105 InterfaceFigure 13 shows an interface between the AD7709 and theADSP-2103/ADSP-2105 DSP processor. In the interface shown,the RDY bit of the Status Register is again monitored todetermine when the Data Register is updated. The alternativescheme is to use an interrupt-driven system, in which case the

REV. A

AD7709

–22–

RDY output is connected to the IRQ2 input of the ADSP-2103/ADSP-2105. The serial interface of the ADSP-2103/ADSP-2105is set up for alternate framing mode. The RFS and TFS pins ofthe ADSP-2103/ADSP-2105 are configured as active lowoutputs and the ADSP-2103/ADSP-2105 serial clock line, SCLK,is also configured as an output. The CS for the AD7709 isactive when either the RFS or TFS outputs from the ADSP-2103/ADSP-2105 are active. The serial clock rate on the ADSP-2103/ADSP-2105 should be limited to 3 MHz to ensure correct opera-tion with the AD7709.

CIRCUIT DESCRIPTION

The AD7709 is a �-� A/D converter with on-chip digital filtering,intended for the measurement of wide dynamic range, lowfrequency signals such as those in weigh scale, pressure, tempera-ture, industrial control, or process control applications. It employsa �-� conversion technique to realize up to 16 bits of no-missing-codes performance. The �-� modulator converts the sampledinput signal into a digital pulse train whose duty cycle containsthe digital information. A Sinc3 programmable low-pass filter isthen employed to decimate the modulator output data stream togive a valid data conversion result at programmable output ratesfrom 5.35 Hz (186.77 ms) to 105.03 Hz (9.52 ms). A choppingscheme is also employed to minimize ADC offset and offset andgain drift errors. The channel is buffered and can be programmedfor one of eight input ranges from ± 20 mV to ± 2.56 V. The inputchannels can be configured for either fully differential inputs orpseudo-differential input channels via the CH2, CH1, and CH0bits in the Configuration Register. Buffering the input channelallows the part to handle significant source impedances on theanalog input, allowing R/C filtering (for noise rejection or RFIreduction) to be placed on the analog inputs if required. Theseinput channels are intended for converting signals directly fromsensors without the need for external signal conditioning. Otherfunctions contained on-chip that augment the operation of theADC include software configurable current sources, switchablereference inputs, and low-side power switches.

The basic connection diagram for the AD7709 is shown inFigure 14. An AD780/REF195, precision 2.5 V reference, providesthe reference source for the part. A quartz crystal or ceramicresonator provides the 32.768 kHz master clock source for thepart. In some cases, it will be necessary to connect capacitors onthe crystal or resonator to ensure that it does not oscillate at over-tones of its fundamental operating frequency. The values ofcapacitors will vary depending on manufacturer specifications.

Analog Input ChannelsThe main ADC has five associated analog input pins (labeledAIN1 to AIN4 and AINCOM) that can be configured as twofully differential input channels (AIN1–AIN2 and AIN3–AIN4)or four pseudo-differential input channels (AIN1–AINCOM,AIN2–AINCOM, AIN3–AINCOM, and AIN4–AINCOM).Channel selection bits CH2, CHI, and CH0 in the ConfigurationRegister detail the different configurations. When the analog inputchannel is switched, the settling time of the part must elapsebefore a new valid word is available from the ADC.

IOUT1

AIN3/P3

AINCOM

AIN1

IOUT2

AIN4/P4

REFIN1(–)

AIN2

REFIN1(+)

REFIN2(–)

REFIN2(+)

RESET

CS

DOUT

DIN

SCLK

P2/SW2

P1/SW1

XTAL1

XTAL2

5V

CHIPSELECT

RECEIVE(READ)

SERIALCLOCK

32.768kHzCRYSTAL

PWRGND GND

AD780/REF195

VIN VOUT

GND

VDD

ANALOG 5VSUPPLY

0.1�F

10�F

0.1�F10�F

ANALOG 5VSUPPLY

AD7709

SERIAL DATA(WRITE)

Figure 14. Basic Connection Diagram

The output of the ADC multiplexer feeds into a high impedanceinput stage of the buffer amplifier. As a result, the ADC inputs canhandle significant source impedances and are tailored for directconnection to external resistive-type sensors like strain gauges orResistance Temperature Detectors (RTDs).

The absolute input voltage range on the ADC inputs when buff-ered (AIN1 to AIN4) is restricted to a range between GND +100 mV and VDD – 100 mV. Care must be taken in setting upthe common-mode voltage and input voltage range so that theselimits are not exceeded; otherwise, there will be a degradation inlinearity and noise performance.

The absolute input voltage range on the ADC inputs whenunbuffered (AINCOM) includes the range between GND – 30 mV toVDD + 30 mV as a result of being unbuffered. The negative abso-lute input voltage limit does allow the possibility of monitoringsmall true bipolar signals with respect to GND.

REV. A

AD7709

–23–

Programmable Gain AmplifierThe output from the buffer on the ADC is applied to the inputof the on-chip programmable gain amplifier (PGA). The PGAcan be programmed through eight different unipolar and bipolarranges. The PGA gain range is programmed via the range bitsin the Configuration Register. With an external 2.5 V referenceapplied, the unipolar ranges are 0 mV to 20 mV, 0 mV to 40 mV,0 mV to 80 mV, 0 mV to 160 mV, 0 mV to 320 mV, 0 mV to640 mV, 0 V to 1.28 V, and 0 to 2.56 V, while bipolar rangesare ±20 mV, ±40 mV, ±80 mV, ±160 mV, ±320 mV, ±640mV, ±1.28 V, and ±2.56 V. These are the ranges that shouldappear at the input to the on-chip PGA.

Typical matching across ranges is shown in Figure 15. Here, theADC is configured in fully differential, bipolar mode with anexternal 2.5 V reference, while an analog input voltage of justgreater than 19 mV is forced on its analog inputs. The ADCcontinuously converts the dc voltage at an update rate of 5.35 Hz,i.e., SF = FFh. A total of 800 conversion results are gathered.The first 100 results gathered with the ADC operating in the±20 mV. The ADC range is then switched to ±40 mV and 100more results are gathered, and so on, until the last 100 samplesare gathered with the ADC configured in the ±2.5 V range. FromFigure 15, the variation in the sample mean through each range,i.e., the range matching, is seen to be on the order of 2 µV.

0 100 200 300 400 500 600 700 800SAMPLE COUNT

AD

C IN

PU

T V

OL

TA

GE

– m

V

19.372

19.371

19.370

19.369

19.368

19.367

19.366

19.365

19.364

ADC RANGE

�20

mV

�40

mV

�80

mV

�16

0mV

�32

0mV

�64

0mV

�1.

28V

�2.

56V

Figure 15. ADC Range Matching

Bipolar/Unipolar ConfigurationThe analog inputs on the AD7709 can accept either unipolar orbipolar input voltage ranges. Bipolar input ranges do not imply thatthe part can handle negative voltages with respect to systemGND. Unipolar and bipolar signals on the AIN(+) input onthe ADC are referenced to the voltage on the respective AIN(–)input. AIN(+) and AIN(–) refer to the signals seen by themodulator that come from the output of the multiplexer, as shownin Figures 16 and 17.

FULLY DIFFERENTIAL

FULLY DIFFERENTIAL AIN(+)

AIN(–)

AIN1

AIN2

AIN3

AIN4

AIN1

AIN2

AIN3

AIN4

MUX

ADC CHANNEL

Figure 16. Fully Differential Mode of Operation

AIN3AIN3

AIN4AIN4

AIN1/AINCOM

PSEUDO-DIFFERENTIALINPUT

AIN(–)

AIN(+)

AIN1AIN1

AIN2AIN2

AINCOMAINCOM

ADC CHANNEL

MUX

AIN2/AINCOM

AIN3/AINCOM

AIN4/AINCOM

PSEUDO-DIFFERENTIALINPUT

Figure 17. Pseudo-Differential Mode of Operation

For example, if AIN(–) is 2.5 V and the ADC is configured foran analog input range of 0 mV to 20 mV, the input voltage rangeon the AIN(+) input is 2.5 V to 2.52 V. If AIN(–) is 2.5 V andthe AD7709 is configured for an analog input range of ±1.28 V,the analog input range on the AIN(+) input is 1.22 V to 3.78 V(i.e., 2.5 V ±1.28 V). Bipolar or unipolar options are chosen byprogramming the UNI bit in the Configuration Register. Thisprograms the ADC for either unipolar or bipolar operation.Programming for either unipolar or bipolar operation does notchange any of the input signal conditioning; it simply changesthe data output coding.

Data Output CodingWhen the ADC is configured for unipolar operation, the outputcoding is natural (straight) binary with a zero differential inputvoltage resulting in a code of 000 . . . 000, a midscale voltageresulting in a code of 100 . . . 000, and a full-scale input voltageresulting in a code of 111 . . . 111. The output code for any analoginput voltage on the ADC can be represented as follows:

Code

AIN GAIN

V

N

REF

=× ×( )

×( )2

1 024.

where:

AIN is the analog input voltage.

GAIN is the PGA gain, i.e., 1 on the 2.56 V range and 128 onthe 20 mV range.

N = 16.

REV. A

AD7709

–24–

When the ADC is configured for bipolar operation, the codingis offset binary with a negative full-scale voltage resulting in acode of 000 . . . 000, a zero differential voltage resulting in acode of 100 . . . 000, and a positive full-scale voltage resulting ina code of 111 . . . 111. The output code from the ADC for anyanalog input voltage can be represented as follows:

Code AIN GAIN VN

REF= ¥ ¥ ¥( )( ) +[ ]-2 1 024 11 / .

where:

AIN is the analog input voltage.

GAIN in the PGA gain, i.e., 1 on the ± 2.56 V range and 128on the ± 20 mV range.

N = 16.

Excitation CurrentsThe AD7709 also contains three software configurable constantcurrent sources. IEXC1 and IEXC2 provide 200 mA of currentwhile IEXC3 provides 25 mA of current. All source currentfrom VDD is directed to either the IOUT1 or IOUT2 pins of thedevice. These current sources are controlled via bits in theConfiguration Register. The configuration bits enable the currentsources, and they can be configured to source current individuallyto both pins or a combination of currents, i.e., 400 mA, 225 mA, or425 mA to either of the selected output pins. These current sourcescan be used to excite external resistive bridge or RTD sensors.

Crystal OscillatorThe AD7709 is intended for use with a 32.768 kHz watch crys-tal. A PLL internally locks onto a multiple of this frequency toprovide a stable 4.194304 MHz clock for the ADC. The modu-lator sample rate is the same as the crystal oscillator frequency.

The start-up time associated with 32.768 kHz crystals is typically300 ms. The OSCPD bit in the Communications Register canbe used to prevent the oscillator from powering down when theAD7709 is placed in power-down mode. This avoids having towait 300 ms after exiting power-down to start a conversion at theexpense of raising the power-down current.

Reference InputThe AD7709 has a fully differential reference input capabilityfor the channel. On the channel, the reference inputs can beREFIN1(+) and REFIN1(–) or REFIN2(+) and REFIN2(–).They provide a differential reference input capability. Thecommon-mode range for these differential inputs is from GNDto VDD. The reference input is unbuffered and thereforeexcessive R-C source impedances will introduce gain errors.The nominal reference voltage, VREF, ((REFIN1(+)– REFIN1(–) or (REFIN2(+) – REFIN2(–)), for specifiedoperation is 2.5 V, but the AD7709 is functional with referencevoltages from 1 V to VDD. In applications where the excitation(voltage or current) for the transducer on the analog input alsodrives the reference voltage for the part, the effect of the lowfrequency noise in the excitation source will be removed

because the application is ratiometric. If the AD7709 is usedin a nonratiometric application, a low noise reference should beused. Recommended reference voltage sources for the AD7709include the AD780, REF43, and REF192. It should also be notedthat the reference inputs provide a high impedance, dynamic load.Because the input impedance of each reference input is dynamic,resistor/capacitor combinations on these inputs can cause dc gainerrors, depending on the output impedance of the source that isdriving the reference inputs. Reference voltage sources like thoserecommended above (e.g., AD780) will typically have low outputimpedances and are therefore tolerant to having decoupling capaci-tors on the REFIN(+) without introducing gain errors in the system.Deriving the reference input voltage across an external resistor, asshown in Figure 18, will mean that the reference input sees asignificant external source impedance. External decoupling on theREFIN pins would not be recommended in this type of circuitconfiguration.

Reset InputThe RESET input on the AD7709 resets all the logic, the digitalfilter, and the analog modulator while all on-chip registers are resetto their default state. RDY is driven high and the AD7709 ignoresall communications to any of its registers while the RESETinput is low. When the RESET input returns high, the AD7709operates with its default setup conditions and it is necessary toset up all registers after a RESET command.

Power-Down ModeLoading 0 to the STBY bit in the ADC Communications Registerplaces the AD7709 in device power-down mode. The AD7709retains the contents of all its on-chip registers (including the dataregister) while in power-down mode.

The device power-down mode does not affect the digital interface,but it does affect the status of the RDY pin. Putting the AD7709into power-down mode will reset the RDY line high. Placing thepart in power-down mode reduces the total current to 26 mAtypical when the part is operated at 5 V with the oscillator runningduring power-down mode. With the oscillator shut down, the totalIDD is 1.5 mA typical at 3 V and 6.5 mA typical at 5 V.

Grounding and LayoutSince the analog inputs and reference inputs on the ADC aredifferential, most of the voltages in the analog modulator arecommon-mode voltages. The excellent common-mode rejectionof the part will remove common-mode noise on these inputs.The digital filter will provide rejection of broadband noise onthe power supply, except at integer multiples of the modulatorsampling frequency. The digital filter also removes noise fromthe analog and reference inputs, provided these noise sources donot saturate the analog modulator. As a result, the AD7709 ismore immune to noise interference than a conventional highresolution converter. However, because the resolution of theAD7709 is so high, and the noise levels from the AD7709 solow, care must be taken with regard to grounding and layout.

REV. A

AD7709

–25–

The printed circuit board that houses the AD7709 should bedesigned such that the analog and digital sections are separatedand confined to certain areas of the board. A minimum etchtechnique is generally best for ground planes as it gives the bestshielding.

It is recommended that the AD7709 GND pin be tied to theAGND plane of the system. In any layout, it is important that theuser keep in mind the flow of currents in the system ensuringthat the return paths for all currents are as close as possible tothe paths the currents took to reach their destinations. Avoidforcing digital currents to flow through the AGND sections ofthe layout.

The PWRGND pin is tied internally to GND on the AD7709.The PWRGND pad internally has a resistance of less than 50 mWto the PWRGND pin, while the resistance back to the GND padis less than 3 W. This means that 19.5 mA of the maximum speci-fied current (20 mA) will flow to PWRGND with the remaining0.5 mA flowing to GND. PWRGND and GND should be tiedtogether at the AD7709, and it is important to minimize theresistance on the ground return lines.

Avoid running digital lines under the device since these willcouple noise onto the die. The analog ground plane should beallowed to run under the AD7709 to prevent noise coupling.The power supply lines to the AD7709 should use as wide a trace aspossible to provide low impedance paths and reduce the effects ofglitches on the power supply line. Fast switching signals likeclocks should be shielded with digital ground to avoid radiatingnoise to other sections of the board, and clock signals shouldnever be run near the analog inputs. Avoid crossover of digitaland analog signals. Traces on opposite sides of the board shouldrun at right angles to each other, which will reduce the effects offeedthrough through the board. A microstrip technique is by far thebest, but is not always possible with a double-sided board. Inthis technique, the component side of the board is dedicated toground planes while signals are placed on the solder side.

Good decoupling is important when using high resolution ADCs.The supply should be decoupled with 10 mF tantalum in parallelwith 0.1 mF capacitors to GND. To achieve the best from thesedecoupling components, they have to be placed as close as possible;chips should be decoupled with 0.1 mF ceramic capacitors to DGND.

APPLICATIONSThe AD7709 provides a low cost, high resolution, analog-to-digitalfunction. Because the analog-to-digital function is provided by a�-� architecture, it makes the part more immune to noisyenvironments, making it ideal for use in sensor measurement andindustrial and process control applications. Given the architectureused in the AD7709, where the signal chain is chopped and thedevice is factory-calibrated at final test, field calibration is notneeded due to the extremely low offset and gain drifts exhibitedby this converter. It also provides a programmable gain amplifierand a digital filter. Thus, it provides far more system-level func-tionality than off-the-shelf integrating ADCs without thedisadvantage of having to supply a high quality integratingcapacitor. In addition, using the AD7709 in a system allows the

system designer to achieve a much higher level of resolution becausenoise performance of the AD7709 is significantly better than thatof integrating ADCs.

The on-chip PGA allows the AD7709 to handle an analog inputvoltage range as low as 10 mV full scale with VREF = 1.25 V. Thedifferential inputs of the part allow this analog input range tohave an absolute value anywhere between GND + 100 mV andVDD – 100 mV. It allows the user to connect the transducerdirectly to the input of the AD7709. The programmable gainfront end on the AD7709 allows the part to handle unipolaranalog input ranges from 0 mV to 20 mV and 0 V to 2.5 Vand bipolar inputs of ± 20 mV to ± 2.5 V. Because the part oper-ates from a single supply, these bipolar ranges are with respectto a biased-up differential input.

Pressure MeasurementOne typical application of the AD7709 is pressure measurement.Figure 18 shows the AD7709 used with a pressure transducer,the BP01 from Sensym. The pressure transducer is arranged ina bridge network and gives a differential output voltage betweenits OUT(+) and OUT(–) terminals. With rated full-scale pres-sure (in this case 300 mmHg) on the transducer, the differentialoutput voltage is 3 mV/V of the input voltage (i.e., the voltagebetween its IN(+) and IN(–) terminals).

Assuming a 5 V excitation voltage, the full-scale output rangefrom the transducer is 15 mV. The excitation voltage for thebridge can be used to directly provide the reference for the ADCas the reference input range includes the supply. Alternatively, asuitable resistor divider can be implemented that allows the fulldynamic range of the input to be utilized in this application.This implementation is fully ratiometric, so variations in theexcitation voltage do not introduce errors in the system. Choosingresistor values of 10 kW and 6 kW as per Figure 18 gives a 1.875 Vreference voltage for the AD7709 when the excitation voltage is 5 V.

AD7709

IN+

OUT+OUT–

IN–

10k�

EXCITATION VOLTAGE = 5V

VDD

AIN1

AIN2

REFIN1(+)

P1

PWRGNDGND

REFIN2(–)

6k�

Figure 18. Pressure Measurement Using the AD7709

Using the part with a programmed gain of 128 results in thefull-scale input span of the AD7709 being 15 mV, which corre-sponds with the output span from the transducer.

REV. A

AD7709

–26–

A second key advantage to using the AD7709 in transducer-basedapplications is that the on-chip low-side power switch can be fullyutilized in low power applications. The low-side power switch isconnected in series with the cold side of the bridge. In normaloperation, the switch is closed and measurements can be takenfrom the bridge. In applications where power is a concern, theAD7709 can be put into low power mode, substantially reducingthe power burned in the application. In addition to this, the powerswitch can be opened while in low power mode, thus avoidingthe unnecessary burning of power in the front end transducer.When taken back out of power-down, and the power switch isclosed, the user should ensure that the front end circuitry is fullysettled before attempting a read from the AD7709.

The circuit in Figure 19 shows a method that utilizes threepseudo-differential input channels on the AD7709 to temperature-compensate a pressure transducer.

5V

OUT(+)OUT(–)

IN(–)

IN(+)

I1

I2

PRESSUREBRIDGE

XTAL1

XTAL2

IOUT1

6.25k�

VDD

REFIN(+)

REFIN(–)

AIN2

AIN1

AIN3

AINCOM

GND

AD7709

250�

Figure 19. Temperature-Compensating a PressureTransducer

In this application, pseudo-differential input channel AIN1/AINCOM is used to measure the bridge output while pseuo-differential channels AIN2/AINCOM and AIN3/AINCOMmeasure the voltage across the bridge. The voltage measuredacross the bridge will vary proportionally with temperature,and the delta in this voltage can be used to temperature-compensate the output of the pressure bridge.

Temperature MeasurementThe AD7709 is also useful in temperature measurement appli-cations. Figure 20 shows an RTD temperature measurementapplication.

REFIN(–)

IOUT1

5V

6.25k�

AIN2

AIN1

AD7709

REFIN(+)

IOUT2

VDD

GND PWRGND

DRDY

SCLK

DIN

DOUT

CS

XTAL1

XTAL2RREFRL1

RL2

RL3

RL4

RCM

RTD

200�A

CONTROLLER

Figure 20. 4-Wire RTD Temperature MeasurementUsing the AD7709

In this application, the transducer is an RTD (Resistive Tem-perature Device), a PT100. The arrangement is a 4-lead RTDconfiguration. There are voltage drops across the lead resistancesRL1 and RL4, but these simply shift the common-mode voltage.There is no voltage drop across lead resistances RL2 and RL3since the input current to the AD7709 is very low, looking into ahigh input impedance buffer. RCM is included to shift the analoginput voltage to ensure that it lies within the common-moderange (GND + 100 mV to VDD – 100 mV) of the ADC. In theapplication shown, the on-chip 200 mA current source providesthe excitation current for the PT100 and also generates the referencevoltage for the AD7709 via the 6.25 kW resistor. Variations inthe excitation current do not affect the circuit since both theinput voltage and the reference voltage vary ratiometrically with theexcitation current. However, the 6.25 kW resistor must have a lowtemperature coefficient to avoid errors in the reference voltageover temperature.

REV. A

AD7709

–27–

Figure 21 shows a further enhancement to the circuit shown inFigure 20. Generally, dc excitation has been accepted as thenormal method of exciting resistive based sensors like RTDs intemperature measurement applications.

IOUT1

IOUT2

VDD

AIN2

AIN1

AIN3

AIN4

AD7709REFIN(+)

MUX1

RREF

A A

BUFANDPGA

200�A

I1

EMF1

RESISTIVETRANSDUCER

EMF2

P1

P2

REFIN(–)

Figure 21. Low Resistance Measurement

With dc excitation, the excitation current through the sensormust be large enough so that the smallest temperature/resis-tance change to be measured results in a voltage change thatis larger than the system noise, offset, and drift of the system.The purpose of switching the excitation source is to eliminatedc-induced errors. DC errors (EMF1 and EMF2) due to para-sitic thermocouples produced by differential metal connections(solder and copper track) within the circuit are also eliminatedwhen using this switching arrangement. This excitation is aform of synchronous detection where the sensor is excited withan alternating excitation source and the ADC measures infor-mation only in the same phase as the excitation source.

The switched polarity current source is developed using theon-chip current sources and external phase control switches (Aand A) driven by AD7709 logic outputs P1 and P2. During theconversion process, the AD7709 takes two conversion results,one on each phase. During Phase 1, the on-chip current source isdirected to IOUT1 and flows top to bottom through the sensorand switch controlled by A. In Phase 2, the current source isdirected to IOUT2 and flows in the opposite direction throughthe sensor and through switch controlled by A. In all cases, thecurrent flows in the same direction through the reference resistorto develop the reference voltage for the ADC. All measurementsare ratiometrically derived. The results of both conversions arecombined within the microcontroller to produce one outputmeasurement representing the resistance or temperature of thetransducer. For example, if the RTD output during Phase 1 is10 mV, a 1 mV circuit-induced dc error exists due to parasiticthermocouples, the ADC measures 11 mV. During the secondphase, the excitation current is reversed and the ADC measures–10 mV from the RTD and again sees 1 mV dc error, giving anADC output of –9 mV during this phase. These measurementsare processed in the controller (11 mV – (–9 mV)/2 = 10 mV),thus removing the dc-induced errors within the system.

In the circuit shown in Figure 20, the resistance measurement ismade using ratiometric techniques. Resistor RREF, which developsthe ADC reference, must be stable over temperature to preventreference-induced errors in the measurement output.

3-Wire RTD ConfigurationsTo fully optimize a 3-wire RTD configuration, two identicallymatched current sources are required. The AD7709, whichcontains two well matched 200 mA current sources, is ideallysuited to these applications. One possible 3-wire configurationusing the AD7709 is shown in Figure 22.

REFIN(–)

IOUT1

GND

5V

6.25k�

AIN2

AIN1

AD7709

RL3

RCM

REFIN(+)

IOUT2

VDD

DRDY

SCLK

DIN

DOUT

CS

XTAL1

XTAL2

RL2RTD

200�A

200�A

RL1

CONTROLLER

Figure 22. 3-Wire RTD Configuration Using the AD7709

In this 3-wire configuration, the lead resistances will result inerrors if only one current source is used since the 200 mA will flowthrough RL1, developing a voltage error between AIN1 and AIN2.In the scheme outlined below, the second RTD current sourceis used to compensate for the error introduced by the 200 mAflowing through RL1. The second RTD current flows throughRL2. Assuming that RL1 and RL2 are equal (the leads wouldnormally be of the same material and of equal length) and thatIOUT1 and IOUT2 match, the error voltage across RL2 equalsthe error voltage across RL1 and no error voltage is developedbetween AIN1 and AIN2. Twice the voltage is developed across RL3but, since this is a common-mode voltage, it will not introduceerrors. RCM is included so the current flowing through thecombination of RL3 and RCM develops enough voltage that theanalog input voltage seen by the AD7709 is within the common-mode range of the ADC. The reference voltage for the AD7709is also generated using one of these matched current sources.

This reference voltage is developed across the 6.25 kW resistoras shown, and applied to the differential reference inputs of theAD7709. This scheme ensures that the analog input voltage spanremains ratiometric to the reference voltage. Any errors in theanalog input voltage due to the temperature drift of the RTDcurrent source is compensated for by the variation in the referencevoltage. The typical drift matching between the two RTD currentsources is less than 20 ppm/∞C. The voltage on either IOUT pincan go to within 0.6 V of the VDD supply.

The AD7709 also includes a 25 mA current source that can be usedalong with the two 200 mA current sources for VBE measurementwhere a 17:1 ratio is required from the current sources.

REV. A

AD7709

–28–

MICROCONTROLLER

VCC

GND

REF OUT2

REF IN

10�F 0.1�F

VDD

AIN1

AIN2

CS

DOUT

SCLK

DIN

GND

REFIN(+)

REFIN(–)0.1�F

REF OUT1

CLOCK

LATCH

DATA

4.7�F

COM

C1 C2 C3

AD7709

AD421

BOOST VCC LV

0.01�F

1k�

1000pF

LOOPPOWER

10�F

3.3V

1.25V

DN25D

0.01�F

LOOPRTN

COMP

DRIVE

VARIABLES

AIN3

AIN4

Figure 23. Smart Transmitter Employing the AD7709

Smart TransmittersSmart transmitters are another key design-in area for the AD7709.The �-� converter, single-supply operation, 3-wire interfacecapabilities, and small package size are all of benefit in smarttransmitters. Here, the entire smart transmitter must operatefrom the 4–20 mA loop. Tolerances in the loop mean that the

amount of current available to power the transmitter is as low as3.5 mA. Figure 23 shows a block diagram of a smart transmitterthat includes the AD7709.

Not shown in Figure 23 is the isolated power source required topower the front end.

REV. A

AD7709

–29–

24-Lead Thin Shrink Small Outline Package [TSSOP](RU-24)

Dimensions shown in millimeters

24 13

1216.40 BSC

4.504.404.30

PIN 1

7.907.807.70

0.150.05

0.300.19

0.65BSC

1.20MAX

0.200.09

0.750.600.45

8�0�

SEATINGPLANE

COMPLIANT TO JEDEC STANDARDS MO-153AD

0.10 COPLANARITY

OUTLINE DIMENSIONS

REV. A

AD7709

–30–

Revision HistoryLocation Page

3/03—Data Sheet changed from REV. 0 to REV. A.

Changes to SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Change to Communications Register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Changes to Table VIII. Filter Register Bit Designations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Updated OUTLINE DIMENSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

–31–

–32–

C02

700–

0–3/

03(A

)P

RIN

TE

D IN

U.S

.A.

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