Sensors Interfacing

8/11/2019 Sensors Interfacing

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SENSORSINTERFACING


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Sensors to ADC

Sensors Output span rarely fit input span of ADC

Offset (a) require level shifting

Unequal span (b) require amplificationBoth (c) Require both level shift andamplification

An OpAmp Level shift and amplify simultaneously


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Interfacing components

OPAMPFiltersComparators

ADCVoltage References


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Op-amp Characteristics

High Input resistanceLow Output resistance

Ability to drive capacitive loadLow input offset voltageLow input bias currentVery high open loop gainHigh common mode rejection ratio


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OPAMP classification criteria

Precision opampSingle/dual supply opampSingle ended/differential opampHigh Bandwidth opampRail to rail IO opamp


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Open loop condition


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Unity gain Voltage follower

Provide impedance conversion from high levelto low level

A follower design should have followingcharacteristics

For current generating sensors input biascurrent of opamp should be at least hundred timesmaller than sensors currentInput offset voltage should be smaller

than required LSB


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Instrumentation Amplifier

Three opamp IA configuration A IA amplifies the difference between V+ andV-


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Instrumentation Amplifiers

IA are available as monolithic ICs Fixed gain range

Easy to set desired gain using a single resistor

Very high CMRR of the order of 100db andmore


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Filters

To remove unwanted signal components in theinput signal

Analog FiltersPassive filters

Designed using passive R,L,C componentsSimple to design 1 st order filters

Active filtersBased on active component like transistors or opampPossible to amplify signal of interest

Digital Filters


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Filter ResponseCharacteristics

Av

ButterworthBesselChebyshev


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Categories of Filters

-3dB {

f 2f

Av(dB)

-3dB {

f 1f

Av(dB)

Low-pass response High-pass response

L ow Pass F ilters :

pass all frequencies from dcup to the upper cutoff

frequency.

H igh Pass F il ters :

pass all frequencies that areabove its lower cutoff

frequency


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Categories of Filters

-3dB {

f 2f

Av(dB)

f 1

-3dB {

f f 2f 1

Av(dB)

Band Pass Response Band Stop Response

Band Pass F il ters :

pass only the frequenciesthat fall between its valuesof the lower and uppercutoff frequencies.

Band Stop (Notch) F ilters :

eliminate all signals withinthe stop band while passingall frequencies outside this

band.


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Single-Pole Low/High-PassFilter

vout-

+

+V

-V

R 1

R f1

R f2

C1

vinvout

-

+

+V

-V

R 1

R f1

R f2

C1

vin

Low Pass Filter High Pass Filter


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DAC

Is a circuit whose output depend on digital inputand associated reference voltageDAC can be implemented using PWM

for PWMVavg=(Ton/T) X Vlh

PWM output filtered using RC filter

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ADC Essentials

Basic I/O Relationship ADC is RationingSystem

x = Analog input /Reference

Fraction: 0 ~ 1

n bits ADCNumber of discrete output

level : 2 n

QuantumLSB sizeQ = LSB = FS / 2 n

Quantization Error1/2 LSB

Reduced by increasing n

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Conversion PARAMETERS

Conversion TimeRequired time (tc) before the converter can provide validoutput dataInput voltage change during the conversion process

introduces an undesirable uncertaintyFull conversion accuracy is realized only if this uncertainty iskept low below the converters resolution

Converter ResolutionThe smallest change required in the analog input of an ADC tochange its output code by one level

Converter AccuracyThe difference between the actual input voltage and the full-scale weighted equivalent of the binary output code

Maximum sum of all converter errors including quantizationerror

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Converting bipolar to unipolar

Using unipolar converterwhen input signal is bipolar

Scaling down theinput

Adding an offsetBipolar Converter

If polarityinformation in outputis desiredBipolar input range

Typically, 0 ~ 5V

Bipolar Output2s Complement

Offset Binary

Input signal is scaled and an offset

is added

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scaled

Addoffset


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Outputs and Analog ReferenceSignal

I/O of typical ADC

ADC output

Number of bits8 and 12 bits are typical10, 14, 16 bits alsoavailable

Errors in reference signal

FromInitial AdjustmentDrift with time and

temperature

CauseGain error in Transfer

characteristicsTo realize full accuracy of ADC

Precise and stablereference is crucialTypically, precision IC voltage

reference is used5ppm/ C ~ 100ppm/ C

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Control Signals

StartFrom CPUInitiate the conversionprocess

BUSY / EOCTo CPUConversion is inprogress

0=Busy: In progress1=EOC: End ofConversion

HBE / LBEFrom CPUTo read Output word after

EOCHBE

High Byte Enable

LBELow Byte Enable

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A/D Conversion Techniques

Counter or Tracking ADCSuccessive Approximation ADC

Most Commonly Used

Dual Slop Integrating ADCVoltage to Frequency ADCParallel or Flash ADC

Fast Conversion

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Counter Type ADCBlock diagram

Suitable for low frequencyhigh resolution conversion

OperationReset and Start CounterDAC convert Digital output

of Counter to Analogsignal

Compare Analog input andOutput of DAC

Vi < V DACContinue counting

Vi = V DACStop counting

Digital Output = Output ofCounter

DisadvantageConversion time is varied

2n Clock Period for Full Scaleinput

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Successive Approximation ADC

Most Commonly used inmedium to high speedConvertersBased on approximating the

input signal with binary codeand then successivelyrevising this approximationuntil best approximation isachievedSAR(Successive

Approximation Register)holds the current binary value

Block Diagram

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Successive Approximation ADC

Circuit waveform

Logic Flow

Conversion Timen clock for n-bit ADCFixed conversion time

Serial Output is easilygeneratedBit decision are made in

serial order

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Parallel or Flash ADC

Very High speed conversionUp to 100MHz for 8 bitresolutionVideo, Radar, DigitalOscilloscope

Single Step Conversion2n 1 comparatorPrecision ResistiveNetworkEncoder

Resolution is limitedLarge number ofcomparator in IC

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Type of ADCs

ADC Resolution Comparison

0 5 10 15 20 25

Sigma-Delta

Successive Approx

Flash

Dual Slope

Resolution (Bits)

Type Speed (relative) Cost (relative)Dual Slope Slow Med

Flash Very Fast High

Successive Appox Medium Fast Low

Sigma-Delta Slow Low

Sensors Interfacing

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