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Sensors Interfacing

Jun 03, 2018

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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

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