1

Circuit Design and Examples

•Design Guidelines–Objectives–Sensor–Signal Conditioning

•Example•Instrumentation Amplifiers

–AD620

•OpAmps–Op07 (modern)–LM741 (ancient)

2

Design Guidelines-1

• Define the measurement objective– Parameter: What do you need to measure:

pressure, temperature, flow, level, etc– Range: What is the range of the measurements?

50-1500 F, 25-65 psi, 100-200 volts– Accuracy: What accuracy is desired and what

specification of accuracy will be used? 5% of Full Scale or 2% of the reading.

– Linearity: Must the measurement be linear?– Noise: How much noise is allowed?

3

Design Guidelines-2• Select the sensor/transducer

– Parameters: What is the input and output of the transducer? E.g. pressure in resistance out, temperature in voltage out, light in current out

– Transfer Function: Output/input relationship?– Time Response: 1st order, 2nd order?– Range: What is the possible range of sensor parameters? 0-

1000C, 3-15 psi, etc– Power: What is the power specification of the sensor?

Sensor Definition in Engineering: the component of an instrument that convertsan input signal into a quantity that is measured by another part of the instrumentand changed into a useful signal for an information-gathering system.

A transducer is an electronic device that converts energy from one form to another. Common examples include microphones, loudspeakers, thermometers, position and pressure sensors, and antenna. Although not generally thought of as transducers, photocells, LEDs (light-emitting diodes), and even common light bulbs are transducers.

4

Design Guidelines-3

• Analog Signal Conditioning– Parameter of output? Voltage, current,

pressure, frequency– Range? 0-5V, 3-15psi, 4-20mA, 300-3500 Hz– Input impedance of the signal conditioning

circuit? Many sensors require a specific impedance input or a range of allowable inputs

– Output impedance to the next stage?

5

Example: Problem 2.33 in textMy solution (check the solution manual for the author’s solution)

2.33: A bridge circuit has R1=R2=R3=120 ohms and V=10.0 volts. Design a signal conditioning system that provides an output of 0 to 5 volts as R4 varies from 120 to 140 ohms. Plot Vout vs R4. Evaluate the linearity

AA B

Desired output is VA-B

6

How to create a graph with

a varying resistance

AA B

1. Place a normal resistor as R4.2. Double click on the value of the

resistance3. Enter {Rvar} [Yes the braces are

required]4. Go to “Get New Part” in the Draw Menu5. Place the part name “PARAM” on the

schematic page6. Double click on Parameters7. Enter Rvar as Name1

And Value1=100

8. Go to Analysis/Setup menu9. Check and open the DC

Sweep box10.Check Global Parameter and

Linear11.Enter the Name of the variable Which is Rvar here, the start and end values of the sweep and the increment for the sweep12. Simulate

10

7VA-VB

)42

4

31

3(10

RR

R

RR

RVV BA

A B

8

VA-VB

A B

0 to .3846 to 0 to 5Requires a gain of5/.3846=13.0

9

Need Differential Gain of 13

10

Instrumentation AmplifiersAnalog Devices Inc. is the largest supplier of instrumentation amplifiers in the world.The AD620 is a low cost, high accuracy instrumentation amplifier which requires only one external resistor to set gains of 1 to 1000. Furthermore, the AD620 offers lower power (only 1.3 mA max supply current), making it a good fit for battery powered, portable (or remote) applications.

The AD620, with its high accuracy of 40 ppm maximum nonlinearity, low offset voltage of 50 µV max and offset drift of 0.6 µV/°C max, is ideal for use in precision data acquisition systems, such as weigh scales and transducer interfaces. The low noise, low input bias current, and low power of the AD620 also make it well suited for medical applications such as ECG and noninvasive blood pressure monitors.

The low input bias current of 1.0 nA max is made possible with the use of Superbeta processing in the input stage. The AD620 works well as a preamplifier due to its low input voltage noise of 9 nV/Hz at 1 kHz, 0.28 µV p-p in the 0.1 Hz to 10 Hz band, 0.1 pA/µHz input current noise. The AD620 is also well suited for multiplexed applications with its settling time of 15 µs to 0.01% and its cost is low enough to enable designs with one in amp per channel.

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AD620 Specifications

http://products.analog.com/products/info.asp?product=AD620

common-mode rejection ratio (CMRR): The ratio of the common-mode interference voltage at the input of a circuit, to the corresponding interference voltage at the output.

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Electrostatic Warning for the AD620 In-Amp

13

AD620 vs opamp

Make vs. Buy: A Typical Bridge Application Error BudgetThe AD620 offers improved performance over “homebrew”

three op amp IA designs, along with smaller size, fewer components and lower supply current. In the typical application, a gain of 100 is required to amplify a bridge output of 20 mV full scale over the industrial temperature range of –40°C to +85°C.

Regardless of the system in which it is being used, the AD620 provides greater accuracy, and at low power and price.

Note that for the homebrew circuit, the OP07 specifications for input voltage offset and noise have been multiplied by 2, because a three op amp type in-amp has two op amps at its inputs.

14Error Budget

15Op07 vs LM741

• The OP-07 has very low input offset voltage (25µV max for OP-07A) which is obtained by trimming at the wafer stage. These low offset voltages generally eliminate any need for external nulling. The OP-07 also features low input bias current (±2nA for OP-07A) and high open-loop gain (300V/mV for the OP-07A). The low offsets and high open-loop gain make the OP-07 particularly useful for high-gain instrumentation applications.

• The wide input voltage range of ±13V minimum combined with the high CMRR of 110dB (OP-07A) and high input impedance provides high accuracy in the non-inverting circuit configuration. Excellent linearity and gain accuracy can be maintained even at high closed-loop gains.

• The OP-07 is available in five standard performance grades.

The LM741 series are general purpose operational amplifiers which feature improved performance over industry standards like the LM709. They are direct, plug-in replacements for the 709C, LM201, MC1439 and 748 in most applications.

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Op07 vs 741

(Inexpensive versions of each)

Op07 (Analog Devices)

LM741 (National Instruments)

Input Offset Voltage

30 to 75 uV 6 to 7.5 mV

Input Offset Current

.4 to 2.8 nA 200 to 300 nA

CMRR 110 dB Min 70 dB Min

Closed Loop BW (gain = 1)

.6 MHz .437 MHz

Slew Rate .3 V/uSec .5 V/uSec

$0.44 for one LM74125 for $8

$1.25 for one Op0725 for $25

From Digikey

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Summary

• Design Guidelines– Objectives– Sensor– Signal Conditioning

• Example• Instrumentation Amplifiers

– AD620

• OpAmps– Op07 (modern) from Texas Instruments, Linear

Technology, or Maxim– LM741 (old but useful) from National Semiconductor

18

Digital Signal Conditioning•AC Bridges

•Number systems

•Boolean Algebra Example

•Tristate Buffers

•Comparators and Circuits

•Schmidt Trigger

•Window Detector

19

AC Bridge Circuits

Generalized AC Bridge

Balanced when:Z1Zx = Z2Z3

A B

20Wien Bridge

)1()

1(

33

321 RCj

RR

CjRR

xx

R1 R2

R3

Rx

C3

Cx

Condition for Balanced Bridge

21

Wien Bridge Oscillator Circuithttp://niuhep.physics.niu.edu/~eads/phys475/lab9.html

.001 uF

.001 uF

10KΩ10KΩ

Adjust the 50K resistorfor a sine wave output

Expected Sine WaveFrequency=15.9 KHz

22

AC Bridges

http://chem.ch.huji.ac.il/~eugeniik/instruments/test/bridges_theory.html

23

Number Systemshttp://www.ibilce.unesp.br/courseware/datas/data1.htm

BITSA bit is the smallest element of information used by a computer. A bit holds ONE of TWO possible values: 0 meaning Off/False/NotSet and 1 meaning On/True/Set

Boolean ValuesBoolean algebra recognizes True and False. So a single bit can represent a Boolean variable.

NIBBLEA nibble is a group of FOUR bits. This gives a maximum number of 16 possible different values. 2 ^ 4 = 16

LSB and MSB:The Least Significant Bit (LSB) is always drawn at the extreme right and has the least value and the Most Significant Bit (MSB) is always shown on the extreme left, and is the bit with the greatest value.

24Number Systems

BYTESBytes are a grouping of 8 bits. This comprises TWO nibbles.

Binary Coded Decimal [BCD]Binary code decimal digits (0-9) are represented using FOUR bits. The valid combinations of bits and their respective values are 0000 through 1001 with the binary combinations 1010 to 1111 not used. If the computer stores one BCD digit per byte, its called normal BCD. The unused nibble may be all 0's or all 1's.

Packed BCD:If two BCD digits are stored per byte, its called Packed BCD. This occurs in data transmission where numbers are being transmitted over a communications link. Packed BCD reduces the amount of time spent transmitting the numbers, as each data byte transmitted results in the sending of two BCD digits.

25

Number SystemsHexadecimal

Refers to the base-16 number system, which consists of 16 unique symbols: the numbers 0 to 9 and the letters A to F. e.g. decimal 15 is represented as F in hexadecimal. This is useful because it can represent a byte (8 bits) as two hexadecimal digits. It is easier to read hexadecimal numbers than binary numbers.

To convert a value from hexadecimal to binary, translate each hexadecimal digit into its 4-bit binary equivalent. Hexadecimal numbers have either an 0x prefix or an h suffix. For example, the hexadecimal number 0x3F7A translates to the following binary number: 0011 1111 0111 1010

http://www.webopedia.com

26

Multiplication/Division

• Multiplication by 10: Shifting left in decimal multiplies by 10. E.g. 05010 50010

• Multiplication by 2: Shifting left in binary multiplies by 2. E.g. 01002 10002 which translates to 410 810

• Division works the same way in that shifting right divides by 10 in decimal, 2 in binary, 8 in octal, and 16 in hexadecimal.

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Push-On Push-Off Control Circuithttp://www.oldradio.com/current/pushon-pushoff.htm

Push-ButtonSwitch

Relay Coil

24 volt dcN.C.

N.O.

28

Example

VisionSystem

D

Wt SensorA

IR SensorB

Robot WelderC

A B C D

0 0 0 0

0 0 0 1

0 0 1 0

0 0 1 1

0 1 0 0

0 1 0 1

0 1 1 0

0 1 1 1

1 0 0 0

1 0 0 1

1 0 1 0

1 0 1 1

1 1 0 0

1 1 0 1

1 1 1 0

1 1 1 1

A

B

D

C

AB

CD

ABCD

Nand

Nand

A + B

C + D

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Tri-state Buffers

http://www.websdeveloped.com/Billy/ComputerOrganization/Assignments/2/Assignment2.htm

1

1

Enable

30

Comparators

http://para.maxim-ic.com/Comparators.htm Maxim

http://www.national.com/catalog/AnalogComparators.html

National Semiconductor

Fairchild Semiconductor

Fairchild

http://amplifier.ti.com

Texas Instruments

31

National Semiconductor LM111 Comparator:

(LM311 is $0.52 each at Digikey)

http://www.national.com/pf/LM/LM111.html

http://www.chipcenter.com/images/tn026.pdf

Article on the meaning of Rail-to-rail

Open-drain outputs are outputs which at any given time are either actively sinking

current (i.e., low voltage, typically considered logic 0) or are high impedance,

but which never source current (high voltage, logic 1). Open-drain refers to the

drain terminal of a MOS FET transistor. The equivalent concept on a bipolar device is

called open-collector.

http://www.brouhaha.com/~eric/pic/open_drain.html

32Comparator Circuit

http://home.cogeco.ca/~rpaisley4/Comparators.html

A

B

33

Zero Crossing Detector

http://www.national.com/ads-cgi/viewer.pl/ds/LM/LM139.pdf

LM339 is $0.52 at Digikey

34

Schmidt Trigger Circuit

2.5 +2.5Sin(2π*100*t)

100 Hz Sinusoid

35Schmidt Trigger

Switches Low Switches High

36Window Detector

http://webug.physics.uiuc.edu/courses/phys344/344exp/Lab1Analog_Digital/Archive/Su2000/lab1.htm#Part_5_Window_Comparator

37

Summary

• AC Bridges

• Number systems

• Boolean Algebra Example

• Tristate Buffers

• Comparators and Circuits

• Schmidt Trigger

• Window Detector