analog to digital converters and data acquisition systems

Principles of Analog to Digital Converters

and

Principles of Data Acquisition

Dr. N. Mathivanan Visiting Professor

Department of Instrumentation and Control Engineering National Institute of Technology,

TRICHY, TAMILNADU INDIA

Data Acquisition Systems

• Data Acquisition (DAQ)

Process of getting digital equivalent of analog signals (the measure

of real world physical quantities) into computer for further

processing

• Data loggers

o Records measurements of physical quantities with time stamp

• Basic Functions of DAQ Systems –

o Analog Input

Conversion of analog signal to digital data and

Transfer of converted data to computing platform using

standard interface N. Mathivanan

o Analog Output

o Digital I/O

o Timing I/O

• Analog Input

o Application: Measurement,

o Prime component: ADC - Characteristics, Types

o Characteristics parameters:

o Other major components:

Analog MUX, PGA, Attenuator, Isolator, Memory

o Sampling methods

N. Mathivanan

Analog to Digital Converter

• Basic Inputs, Outputs

Vin Analog input, ‘n’ Digital Output, SoC input, EoC output, Vref

• Characteristic parameters

Resolution, R = VFS/(2n – 1)

Input-Output relation: D = Vin / R

Conversion time: Time the ADC takes to produce a valid binary

output for an applied analog input after the conversion is initiated

EOC

Vin

SOC

comparator

Vref

digital output

D0control logic

controlregister

D1

DAC

analoginput

D2

SOC

D

A

D

C

EOC

n-1 110

7/8

111

1/8

011

010

2/8 3/8

001

digitaloutput

4/8

000

5/8

analog input

codewidth

6/8

1 LSB

0

100

FS

101

N. Mathivanan

ADC Types

• Integration type

o Two types: single slope, dual slope

o Principle: time taken to charge / discharge the applied input voltage

in terms of count values of a counter

o Characteristics: Speed low, rejects noise, cheap, available in high

resolutions

in 1 in 2

(V )

time

discharge

(fixed rate)

in 1

T1

2in

integration

T

(V )

(V ) > (V )

2N. Mathivanan

• Successive approximation type

o Principle: Equivalent to determining the unknown weight of an

object using standard weights

o Characteristics: Speed medium to high (Conversion time = n+1

clock periods), cannot reject noise, S/H device required, cost

high, available in high resolutions

MSB

EOC

comparator

in

SAR Register

SOC

V

digitaloutput

LSBo

clock

V

DAC

FS

2

010

clock

001

3

compare D0

o

in

compare D1

compare D2

7 x LSB

V

011

6 x LSB

111

5 x LSB

110

4 x LSB

101

3 x LSB

001

011

111

V = 6.5 V

2 x LSB

110

1 x LSB

0 x LSB

100

101

100

1

010

000N. Mathivanan

• Parallel converter/Flash converter

o Principle: Unknown input is compared

with diff. discrete V levels and

encoded

o Characteristics: High speed, High

cost, No need for S/H, only low

resolution

R

1

+

R / 2

6

_

_

R

D

+

+

D

4

3

R

D

V

R

2

+

priorityencoder

in

+

_1.875 V

R

6.875 V

5

_

5.625 V

1

7

3R / 2

0.625 V

R

_

3.125 V

V = 10.0 V

8.125 V

analog input

ref

_

+

_4.375 V

0

2+

N. Mathivanan

Comparing ADCs

Characteristics Integration Successive

Approximation Flash ADC Sigma-Delta

Speed Slow Fast Fastest Slow

Noise Rejects Not eliminated Not

eliminated

S/H device Not required Required Not

required Not required

Resolution High Medium to High Low

resolution High

resolution

Cost Low Modest High Low

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AD574A ADC

CONTROL

9

N

I

B

B

L

E

C

17

S A R

+5 V

10 V

5 k

CE# 6

10 VRef

14

STS

A

5

3k

DB0

8k

16

26

Dig ita lCom mon

5 k

DB8

EE

CLOCK

15

DAC

9.95k

4

12

Vcc

13

20

_BIP OFF

27

DB1

28

DB3

V

EE

I = 4 x N x I

MSB

REF

8

7

12

22

21

COMP

23

+

+

DB9

LSB

1

18

25

11

REFDAC

20 V

in

10

DB2

_

REF OUT

12

DB4

REF IN

DB6

DB5

DB7

-12 / -15 V V

12

3

DB11

AnalogCom mon

N

I

B

B

L

E

A

o

I

19.95k

in

19

N

I

B

B

L

E

B

12/8# 2

DB10

24R/C# 3

S

T

A

T

E

O

U

T

P

U

T

B

U

F

F

E

R

S

CS#

12-bit/8-bit ADC

10V/20V Range

Unipolar/Bipolar

Interfacing to 8-bit/

16-bit bus

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Sampling Concepts

• Sampling o Quantizing amplitude of continuous signal to digital data at

discrete times

• Samples

o Series of data obtained by sampling are the samples

o Samples cannot represent and process the original signal without

error

• Sampling rate

o No. of samples collected in one sec.

• Sampling theorem

o Relates sampling rate & max freq. component in the signal

o fS > 2 fH

N. Mathivanan

Sampling

amplitudequantization

t

discrete time sampling

continuous

analog

signal

y ( t )

4T 9T 11T5T 12T 14T10TT0 13T2T 6T3T 8T7T 15T16T

(b)

(a)

(c)N. Mathivanan

Discrete Time Signal

0

x ( t )

t

(a)

14T12T13T8T5T 10T11T3TT

p ( t )

7T4Tt

9T6T0 2T

(b)

x ( t ) x ( t )p

p ( t )

(c)

4T 9T5TT 10T 13T8T 11T3T 12T 14T6T2T0 7T

(d)

px ( t )

t

N. Mathivanan

Frequency Spectrum

P ( f )

f

X ( f )

S

f2fS 0

f

H

-2f

-f

S

H

f-fS

H

ffH-f fS S2fS-f-2fS

(c)

(b)

(a)

Xp ( f )

low-pass filter

N. Mathivanan

Aliasing X ( f )

f-f AA

C

S- f + f- f -fA

A

f + f0S

B'C'

Xp ( f )

A f / 2S A

f f

B

Nyquistbandwidth

S A

A'

S S- f / 2 f - f

S A

A - f + f f- f - f -fS A

f + fS A

A'

S A- f

C

f - f

B'

f

A B

S A

C'

AS S

f / 2SS

- f / 2

Xp ( f )

(a)

(b)

(c)

S A- f - f

N. Mathivanan

Oversampling & Undersampling

f / 2S

Hf

Sf

S Hf - f

(a)

X ( f )

f

fH

f / 2S

f - fS H

fS

(b)

X ( f )

f

N. Mathivanan

Effect of Amplitude Quantization

codewidth

analog input

110

1/8

- Q/2

111

0 3/8

(a)

2/8

000

6/8

010

1 LSB

5/8

analog input

7/8

001

4/8

011

(b)

101

+ Q/2

FS

digitaloutput

100

• Quantization noise

• For full-scale sine wave to n-bit ADC, the SNR = (6.02 n + 1.76) dB

• For one bit increase SNR increases by 7.78 dB

• By increasing sampling freq by a factor of k and filtering noise,

The SNR improves to, SNR = [6.02 n + 1.76 + 10 log10 (k)] dB

fA S

signal

S

frequency bandof interest

quantization noise

fS

amplitude

average noiselevel

S

amplitude

f / 2

f / 2 k f / 2

A

signal

frequency bandof interest

average noiselevel

f S

k fN. Mathivanan

Sigma-Delta Converter

• Sigma-Delta converter

o Delta-sigma / oversampling / noise-shaping

o Principle: uses oversampling, noise shaping, digital decimation &

filtering

o Characteristics: High resolution, low cost, low speed

o Used in: professional audio systems, high precision measurement

systems

• Sigma-Delta Modulator

o Difference Amplifier, Integrator, Comparator (1-bit ADC),

o SPDT switch (1-bit DAC) in the feedback path

• Digital decimator

o Performs down sampling of data stream & produces n-bit output N. Mathivanan

Sigma-Delta Modulator and Oversampling

N. Mathivanan

Conversion Sequence

• Range: -Vref to +Vref.

• Vin = +(5/8) Vref

• Any 4-bit ADC operating in the

above range, converts the above

Vin to digital value 13.

• Averaging (downsampling) 16 values

yield 4-bit resolution ADC.

• Averaging 32 values give 5-bit res

Sample N`

X Input

B (X-Wn-1)

C (B+Cn-1)

D (0 / 1)

W (+1 /-1)

0 5 /8 0 0 0 0

1 5 / 8 5 / 8 5 / 8 1 +1

2 5 / 8 -3 / 8 2 / 8 1 +1

3 5 / 8 -3 / 8 -1 / 8 0 -1

4 5 / 8 13 / 8 12 / 8 1 +1

5 5 / 8 -3 / 8 9 / 8 1 +1

6 5 / 8 -3 / 8 6 / 8 1 +1

7 5 / 8 -3 / 8 3 / 8 1 +1

8 5 / 8 -3 / 8 0 / 8 0 -1

9 5 / 8 13 / 8 13 / 8 1 +1

10 5 / 8 -3 / 8 10 / 8 1 +1

11 5 / 8 -3 / 8 7 / 8 1 +1

12 5 / 8 -3 / 8 4 / 8 1 +1

13 5 / 8 -3 / 8 1 / 8 1 +1

14 5 / 8 -3 / 8 -2 / 8 0 -1

15 5 / 8 13 / 8 11 / 8 1 +1

16 5 / 8 -3 / 8 8 / 8 1 +1

17 5 / 8 -3 / 8 5 / 8 1 +1 N. Mathivanan

Modulator Output

• Modulator output represents

digital value for the input

• For 3 different analog inputs

o Vin = + (Vref/2) V

o Vin = 0 V

o Vin = - (Vref/2) V

• Compare integrator output

waveforms for the 3 inputs

• Compare output waveforms of

comparator for 3 inputs

N. Mathivanan

• Oversampling

o For full-scale sine wave to n-bit ADC, the SNR = (6.02 n + 1.76) dB

o For one bit increase SNR increases by 7.78 dB

o By increasing sampling frequency by a factor of k and filtering noise,

o The SNR improves to, SNR = [6.02 n + 1.76 + 10 log10 (k)] dB

o Oversampling by 4 times improves SNR by 6 dB

o Max. sampling rate limited & is set by ADC

Principles of Operation – Oversampling & Noise Shaping

N. Mathivanan

• Noise Shaping

o Freq. Spectrum is shaped – Max. noise is pushed to high freq side

o Sigma-Delta modulator does this job – Linear model explains

o Integrator: analog filter with transfer function H(f)

o Comparator (quantizer) : amplifier (gain 1) + noise adder

o Output ‘y’ vs. input ‘x’ is given by an expression:

𝒚 =𝒙+𝒚

𝒇+ 𝒒, rearranging, 𝒚 =

𝒙

𝒇+𝟏+

𝒒𝒇

𝒇+𝟏

o At low frequency, output ‘y’ has only signal component ‘x’

o At high frequency, ‘y’ has noise component and less ‘x’

N. Mathivanan

• Characteristics

o Digital filter uses previous values also for generating outputs.

o Beyond specified range, decimator clips off digital output

o With mux input, switching requires flushing off all information

• Advantages:

o Easy integration into CODECs, mC, DSP chips, S/H not required

o Requirement of anti-aliasing min, noise level independent of sig

• Disadvantages:

o Limited to high-resolution & low frequency applications

o Not suitable for multiplexed, fast varying signals

• Application Examples:

o MAX1120 - Thermocouple DAS uses 24-bit ∑-δ ADC, USB

o Smart 4-20 mA Transmitter (HART) uses 24-bit ∑-δ ADC N. Mathivanan

• Second order Sigma-Delta Modulator:

o Uses two difference amplifiers and two integrators

o Improves SNR by 15 dB for every doubling of sampling rate

• Sigma-Delta DAC

• Reverse process, has similar advantages like ADC

• Not popular because it is expensive and high precision, R-2R

ladder network based cheap DAC are available. N. Mathivanan

DAQ Systems – Functional Blocks

• Specification Parameters

o Channels: Single / Multi Channel, No. of Channels

o Input type: Single ended or Differential

o Input Range: Range of operation of ADC, PGA, Attenuation

o Resolution: No. of bits in ADC output

o Throughput: Conversion time of ADC

o Isolation:

To provide protection to both application side and system side

To protect differential amplifiers from large common mode voltages

N. Mathivanan

• Single / Multichannel Analog Input stage of a DAQ

o DAQ steps:

(i) Issue SoC, (ii) Monitor EoC, (iii) Get converted data

o If Successive Approximation type ADC is used:

(i) Trigger S/H to ‘HOLD’ mode

(ii) Issue SoC, (iii) Monitor EoC, (iv) Get converted data

(ii) Trigger S/H back to ‘SAMPLE’ mode

o Choice of ‘HOLD’ capacitor value is critical (discussed later)

o Vin to ADC should be within the range of ADC

Provide gain/attenuation selection to handle different ranges

Low level inputs are amplified to optimum level using PGA

o Multiplexer (analog) allows multichannel sampling

Single-ended input uses 1 MUX and Differential input uses 2 MUX

N. Mathivanan

• Single ended, Single/Multi Channel input types and PGA

o Non-inverting Amplifier Gain, 𝐴 = 𝑉𝑖𝑛 1 +𝑅2

𝑅1

o Simple programmable gain amplifier design using analog mux CD4052

Gain = 1, 10, 100, 1000

o Attenuator, 1:10 attenuation, protection,

o Multichannel inputs using analog MUX

N. Mathivanan

• Differential, Single / Multichannel Input and Instrumentation Amp

• If R1=R3=R & R2=R4=Rf, differential amp gain is 𝐴 =𝑉𝑜

𝑉𝑖𝑛+ −𝑉𝑖𝑛(−)

=𝑅𝑓

𝑅

• Gain of Instrumentation Amplifier, 𝐴 = 𝑉2− 𝑉1 1 +2

𝑎 where 𝑎 =

𝑅𝐺

𝑅

N. Mathivanan

• Sample and Hold Amplifier

o Two op-amp based buffer amps (A1 & A2), high-speed transistor

switch (SW) and capacitor (CH)

o ‘1’ applied to SW switches S/H to ‘Hold’ and ‘0’ to ‘Sample’ mode

o In ‘Sample’ mode CH charges to level of Vin and appears at output,

i.e. output tracks input

o In ‘Hold’ mode CH retains Vin captured just before driven to Hold

o Criteria for choosing value for CH

High value has large acq time

Low value has large droop

Value that has drooping less than 1 LSB

in one conversion is selected

N. Mathivanan

• Multiplexing input

o Sequential sampling

Time multiplexed sampling

There is time skew between two channels

Phase relationship can’t be analyzed

o Simultaneous sampling

All inputs are simultaneously captured and

Sequentially sampled

(Compute phase shift in sampling 10 kHz signal

applied in Ch1 and Ch4 and sampling at 100 kHz

Sampling rate)

N. Mathivanan

• Sample timing

o Real-time sampling

o Equivalent-time sampling

N. Mathivanan

N. Mathivanan

Isolation

• Isolation Uses

o Applications that require detection of low level differential

signals in the presence of high level noise, interference or common

mode voltages

o Applications that require mutual protection to sensor circuits and

measurement circuits from possible damage that may be caused

by ground defects or high voltages at one circuit on the other.

o Applications that require very high impedance path between

different grounds to avoid interference currents

N. Mathivanan

Analog Isolation

• Magnetic Isolation o Both input & output circuits use

separate grounds

o Isolation by magnetic coupling

o DC signals can’t be transmitted

• Opto-Coupler

• LED emitter & phototransistor detector

• Transmits signals alone between circuits having different grounds

N. Mathivanan

• Capacitive coupling

o High frequency carrier signal is frequency or PWM modulated

with signal

o Capacitively coupled with output stage,

o At output stage, signal is demodulated and filtered.

o Suitable for low isolation voltages

N. Mathivanan

Noise & Noise Reduction

• Externally generated noise

o Generated externally and enters into the system along with signal

• Internally generated noise

• Generated by internal component due to ageing, etc.

• Drift in characteristics

• Induced noise

• Picked up by the circuit through resistive, capacitive and inductive

coupling

• Noise induced by: improper grounding, EMI, RFI

N. Mathivanan

Grounding

• Improper and proper grounding

• Grounding in mixed signal systems

N. Mathivanan

Shielding and Shield Grounding

• Electromagnetic interference (EMI)

• Minimizing electric field interference

• Shield grounding

N. Mathivanan

Filtering

• RF Interference

o Filtering

• Power supply

• Amplifier inputs

• Amplifier outputs

N. Mathivanan

I/O Techniques

• Programmed I/O (Polling)

• Interrupt driven I/O

• Buffered I/O

o Double buffer

o Circular buffer

o FIFO buffer

• DMA

N. Mathivanan

Sampling Methods

• Software polling

• Clocked sampling

• Timer provides timing signals to initiate SoC

• Uses on-board Timer or system Timer

• External sampling

• Auto-sampling

• Multi-rate sampling – capturing transient signals

N. Mathivanan

Analog Output

• Applications

o Waveform generation

o Speed control of dc motor

• DAC

o Settling time

o Current to voltage converter

o Drivers

o DAC for ADC converter (low cost systems)

N. Mathivanan

Digital I/O

• Applications

o Speed control of stepper motor

o Control of on/off switches

• Latches / Buffers

– 74LS273, 74LS373, 74LS374

• Programmable devices

o 8255

• Drivers

• Digital isolation and surge protection

N. Mathivanan

Timing I/O

• Provides timing signals for all applications

• Uses

o Frequency / period / pulse width measurements

o Event counting, interval timing, speed monitoring

o Time base or pulse generation

o Frequency measurement

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Other Considerations

• Bus buffering

• Signal grounds

• Power decoupling

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PC Bus based Data Acquisition System

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Block diagram

Reference

• PC Based Instrumentation: Concepts and Practice,

N. Mathivanan, PHI Learning, V Printing, 2014

N. Mathivanan