chapter_3_-_signal_conditioning.pdf

8/14/2019 Chapter_3_-_Signal_Conditioning.pdf

1/13

1

CHAPTER3

SIGNALCONDITIONING

2

Objectives

Attheend

ofthechapter,studentshouldb

eableto:

Unders

tandthepurposeandoperation

s

involve

dinperforminganaloganddigitalsignal

conditionings.

Unders

tandthedifferencesbetweenanalogand

digitalsignalconditionings

Describesomeofthemainoperations

inanalog

anddig

italsignalconditionings.

3

Introduction

The

information

or

data

generated

by

a

basic

measuringdev

icegenerallyrequireprocessing

or

conditioningofonesortoranotherbeforetheya

re

presentedtoth

eobserverasanindicationorarecord.

Signalconditio

ning

circuitsimprove

the

quality

of

signals

gener

ated

by

sensors

before

theya

re

convertedintosuitablesignal,e.g.digitalsignal.

Signalconditio

ningcircuitalsoprovideprotection

to

measurement

systemsfrom

possible

damagethat

maybecaused

byaccidentalhighvoltage,surge,etc.

4

Princip

leOfSignalConditioning

Signalcon

ditioninginmeasurementsystemperformsn

ecessary

operations

onsensoroutputsandconditionsthesignaltoaform

andleveln

ecessarytointerfacewithothercomponentsofthe

measurem

entsystem.

Forexamp

le:-

Thevo

ltagegeneratedbyathermocouple,usedas

sensorina

temperaturemeasurementsystemisverylowinlev

elandweak

inpower.

Hence,theoutputofthesensorneedstobeconditionedby

proper

signalconditionere.g.instrumentationamplifier.


2/13

5

SignalCon

ditioningProcesses

Thefollowingare

someoftheprocessesthatcan

occurincondition

ingasignal:-

1.

Protectiontopre

ventdamagetothenextelement.Eg

;

amicroprocesso

r,duetohighcurrentorvoltage,

componentused

maybeseriescurrentlimitingresistor,

fuse,polaritypro

tectionandvoltagelimitationcircuit.

2.

Gettingthesignalintotherighttypeofsignal.Itmean

s

makingthesignalintod.c.voltageorcurrent.Eg;

resistancechangeofastraingaugehastobe

convertedintoa

voltagechange.(Wheatstonebridge

)

6

SignalC

onditioningProcesses

(cont.)

3.

Gettingthelevelofthesignalright.Thesignalfrom

thermoco

upleisjustafewmilivolts.Ifthesignalistofed

intoanA

DCasinputtouP,itneedstobe

madeinto

muchlargervolts.(Op-amp)

4.

Eliminate

orreducingnoise.Signalmaybesu

sceptibleto

electromagnetic

radiation

which

causesn

oise.Thus

noiseneedtoberemoved.(filter)

5.

Signalm

anipulation,e.g.makingitalinear

functionof

somevariables.(flowmeter)

7

SignalCon

ditioningOperations

Signalconditioningcircuitsaredesignedbasedon

sensor,

signal

type,

signal

level,

frequency,

environmentand

distance.

Signalconditioningcircuitsimprovethequalityof

signalsgenerate

dbytransducersbeforetheyare

convertedintodigitalsignalsbythePC'sdata-

acquisitionhardw

are.

Examplesofsignalconditioningaresignalscaling,

amplification,

linearization,

cold-junction

compensation,

filtering,

attenuation,

excitation,

common-moderejection,andsoon.

8

An

alogSignalConditioning

Analogsignalconditioning

means

manipula

ting

an

analog

sig

nalin

such

a

way

thatitme

ets

the

requiremen

tsofthenextstageforfurtherproces

sing.

Itisprimarilyutilizedfordataacquisition,inwhic

hsensor

signalsmustbenormalizedandfilteredtolevels

suitable

foranalog-to-digitalconversionsotheycanbe

readby

computerizeddevices.

Theoutput

fromtheanalogsignalconditioningisstillan

analogsign

al.


3/13

9

Analogsignalcond

itioningcircuitmayconsistsofanyof

thefollowingcomponents;

Buffering:basicandstraightforwardsignalconditionin

g

circuit,op-ampIC

isanexampleofgoodbuffer.

Filtering:unwantedsignalornoiseespeciallyin

industrialapplicationisreducedoreliminatedusing

properfilteringcirc

uit.Combinationofresistors,

capacitors&induc

torsarecalledpassivefilterswhile

usinganop-ampw

ithgain&feedbackiscalledactive

filter.

AnalogSignalConditioningComponen

ts

10

Modulator:Usesacamplifiertochangedc

signalto

pulses/squarewavesignal.Therearefewt

ypesof

modulato

rusedinsignalconditioningcircuit.

Linearization:Anon-linearoutputfromtran

sducers

mustbelinearizedbyusingamplifier.

AnalogSignalConditioningComponents

Amplifiers:Op-ampisanexampleofamplifie

rthatcan

performs

umming,integration,differentiationa

nd

comparator.Thecommontypesofamplifierar

enon-

inverting

op-amp,andinvertingop-ampwithfe

edback.

11

Electricalsignals

areconditionedsotheycanbeused

by

analoginputboard.

Someanalogsignalconditioningoperationsare:-

1.

Buffering

2.

Filtering

3.

Amplification

4.

Modulator

5.

Linearization

However,onlyam

plificationandfilteringarenormally

requiredtoprepa

rethesignalforconversionintodigital

form.

CommonAnalogSignal

ConditioningOperations

12

Buffering

Manytransduc

ershavealimitedcurrentcapabilityandmustbeoperatedintoa

highimpedanc

e.SobufferamplifierItisrequiredformatchingimpedancesand

thusreducingtheloading.Itisacircuitwhichtransformselectricalimpedancefrom

onecircuittoa

nother.Itsmainpurposeistopreventtheloadingof

apreceding

circuitbythesucceedingone.Abasicandstraightforwardsignalco

nditioning

circuit,op-amp

ICisanexampleofagoodbuffer.

Forexample,a

sensormayhavethecapabilitytoproduceavoltageorcurrent

corresponding

toaparticularphysicalquantityitsensebutitmayn

othavethe

powertodrive

circuitryitisconnectedto.Insuchsituationsabuffercanbe

used.Abufferwhenconnectedbetweenthesensorandthesuccee

ding

circuitryeasily

drivesthecircuitryintermsofcurrentorvoltageaccordingto

thesensoroutput.

Buffersareclassifiedintovoltagebuffersandcurrentbuffers:

Voltagebuffer-Acircuitwhichtransfersavoltagefromacircuitwithhigh

outputimpedancetoacircuitwithlowinputimpedance.

Currentbuffer-Currentbufferisacircuitthatisusedtotransfercurrentfroma

lowinputimpedancecircuittoacircuithavinghighinputimpedance.


4/13

13

Filtersarecircu

itsthatarecapableofpassing

signalswithina

bandoffrequencieswhile

rejectingorblockingsignalsoffrequencies

outsidethisband.

Thispropertyo

ffiltersisalsocalledfrequency

selectivity.

Sincetheoutpu

tlevelofsensorsisverylow,it

is

easilysusceptibletoelectromagneticnoise.

Hence,simpleamplificationisnotsufficientand

afteramplificationitisnecessarytoeliminate

noisefromthesignal.

Filtering

14

Filtering

Filtera

reusedfortheeliminationofno

ise.

Primar

ypurposeoffiltersistoremove

signal

conten

tatunwantedfrequencies.

Filters

canbepassiveoractive.

Passiv

efiltercomprisedofresistors,capacitors,

andinductorsthatrequirenoexternal

supply.

Active

filtersuseresistorsandcapacitorswith

operationalamplifiers,whichrequirespower.

15

Advantage

sOfActiveFiltersOver

P

assiveFilters

Activefilters

canbedesignedtoprovide

requiredgain,andhencenoattenuationas

inthecaseo

fpassivefilters

Noloadingp

roblem,becauseofhighinput

resistanceandlowoutputresistanceofop

-

amp.

ActiveFiltersarecosteffectiveasawide

varietyofeconomicalop-ampsare

available.

16

BasicfilterResponses

Afilterisacircuitthatpassescertainfrequenciesand

rejectsallo

thers.Thepassbandistherangeof

frequencies

allowedthroughthefilter.Thecritical

frequency

definestheend(orends)ofthepass

band.

Basicfilter

responsesare:

f

Gain

f

Gain

f

Gain

f

Gain L

ow-pass

High-pass

Band-pass

Band-stop


5/13

17

TheBasicLow

-PassFilter

Thelow-passfilterallowsfrequenciesbelowthe

criticalfrequency

topassandrejectsother.The

simplestlow-pas

sfilterisapassiveRCcircuitwith

theoutputtaken

acrossC.

f

BW

0dB

20dB

10fc

40dB

60dB

0.1

fc

fc

0.0

1fc

100fc

1000fc

Passband

3dB

Gain(normalizedto1)

Actualresponseofa

single-poleRCfilter

Transition

region

Stopband

region

20

dB/decade

Vout

R

Vs

C

18

TheBasicHigh-PassFilter

Thehigh-passfilterpassesallfrequenciesabovea

criticalfrequencyandrejectsallothers.Thesimplest

high-passfilterisapassiveRCcircuitwiththe

output

takenacrossR.

f

0dB

20dB

fc

40dB

60dB

0.01fc

0.1

fc

0.001fc

10fc

100fc

Passband

3dB

Gain(n

ormalizedto1)

Actualresponse

ofasingle-pole

RCfilter

20dB/dec

ade

Vout

R

Vs

C

19

TheBand-PassFilter

Aband-passfilterpassesallfrequenciesbetween

twocriticalfreque

ncies.Thebandwidthisdefinedas

thedifferencebetweenthetwocriticalfrequencies.

Thesimplestband-passfilterisanRLCcircuit.

Vout

R

Vs

C

L

0.7

071V

out

(normalizedto1)

BW

fc1

f0

fc2

f

20

TheBa

nd-StopFilter

Aband-s

topfilterrejectsfrequenciesbetwee

ntwo

criticalfrequencies;thebandwidthismeasured

between

thecriticalfrequencies.Thesimplest

band-

stopfilterisanRLCcircuit.

Vout

R

Vs

CL

3

Gain(dB)

fc1

f0

fc2

f

0

BW


6/13

21

ActiveFilters

o

Activefiltersincludeo

neormoreop-ampsinthedesign.

o

Activefiltersaremain

lyusedincommunicationandsignal

processingcircuits.

o

Thesefilterscanprov

idemuchbetterresponsesthanthe

passivefiltersillustrated.Activefilterdesignsoptimizevarious

parameterssuchasa

mplituderesponse,roll-offrate,or

phaseresponse.

Av

f

Butterwo

rth:flatamplituderesponse

Chebyshev:rapidroll-offcharacte

ristic

Bessel:linearphaseresponse

Filterswithaflatpass-bandgain

arecommonlyused,and

sucha

responseisprovidedby

Butterworthfilters.Ananother

classoffilterscalledchebyshev

filters,providearipple(or

overshootsin)

pass-bandgain.

22

ActiveFilters

Addingca

pacitorstoop-ampcircuitsprovidesexternal

controlof

thecutofffrequencies.Theop-ampa

ctive

filterprovidescontrollablecut-offfrequenciesa

nd

controllab

legain.

Low

-passfilter

High-passfilter

Ban

dpassfilter

Ban

d-stopfilter

Eachofthesefilterscanbebuiltusingop-amp

asthe

activeele

mentandresistorsandcapacitorsas

the

passivee

lements(frequencyselectivepart).Betterfilter

performanceisobtainedbyemployingop-amp

swith

highersle

wratesandhighergain-bandwidths.

23

1

1

OH

C

R2

1

f

====

1f

v

RR

1

A

++++

====

Low-PassFilter

Theuppercutofffre

quency

andvoltagegainare

given

by:

24

High-PassFilter

1

1

OL

C

R2

1

f

====

Thecutof

ffrequencyisdeterminedby:


7/13

25

B

and-PassFilter

Therearetwocutoff

frequencies:upperand

lower.Theycanbe

calculatedusingthesame

low-passcutoffandhig

h-

passcutofffrequency

formulasinthe

appropriatesections.

26

Amplifiers

Oneo

fthe

mostcommon

signalconditioning

functionsisamplification.

Forma

ximum

resolution,thevoltagerangeofthe

inputsignalsshouldbeapproximatelyequaltothe

maximuminputrangeoftheA/Dconverte

r.

Amplificationexpandstherangeofthetransducer

signals

sothattheymatchtheinputrangeofthe

A/Dconverter.

Forex

ample,a

x10

amplifiermaps

transducer

signals

whichrangefrom

0to1Vintoth

erange0

to10V

beforetheygointotheA/Dconve

rter.

27

Amplifiers

Signalamplificationiscarriedoutwhenthetypical

signallevelofameasurementtransduceris

consideredtobe

toolow.

Amplificationbyanalogmeansiscarriedoutbyan

operationalamplifier.

Normallyrequirestohaveahighinputimpedanceso

thatloadingeffec

tonthetransduceroutputsignalis

minimized.

Whenamplifying

theoutputsignalfromaccelerome

ters

andsomeopticaldetectors,theamplifiermusthavea

highfrequencyresponse,toavoiddistortionofthe

outputreading.

28

Amplifiers

Mostse

nsorsproduceverylowlevelsignalsinthe

formofvoltage,currentorresistance.

Ifthe

outputofsensoris

V

orI,the

signal

conditionerwilluseavoltage@

currentam

plifier.

Iftheoutputofasensorisachangeinresistance,

thenabridgecircuitisemployedfordetectingthe

changes

inresistanceandabridgeamplifierisused

foramp

lifying

bridge

outputsand

improving

the

sensitivityofdetection.


8/13

29

Operationalamplifierorop-amp,isaveryhighgain

differentialamplifierwithhighinputimpedance(typicallya

few)andlow

outputimpedance(lessthan100W).

Opampistheba

siccomponentinanalogsignal

conditioningcircu

it.

Canperformman

yoperationslikeamplification,addition,

subtraction,integ

ration,differentiation,etc

Notetheop-amp

hastwoinputsandoneoutput.

B

asicOp-Amp

30

InvertingOp-Amp

Thesign

alinputisappliedtotheinverting

()input.

Thenon

-invertinginput(+)isgrounded.

TheresistorRfisthefeedbackresistor.Itis

connectedfromtheoutputtothenegative

(inverting)

input.Thisisnegativefeedback.

31

Inve

rtingOp-AmpGain

Gaincanbedeterm

ined

fromexternalresistors:Rf

andR1

Unitygainvoltage

gainis1

Thenegativesign

denotesa180

phaseshiftbetwee

n

inputandoutput.

1f

io

v

RR

VV

A

=

=

1

RR

A

R

R

1f

v

1

f

====

========

ConstantGain-RfisamultipleofR1

32

Inv

erting/NoninvertingOp-A

mps

1

1f

o

V

RR

V

====

Inv

ertingAmplifier

NoninvertingAmplifier

1

1f

o

V)

RR

1(

V

++++

====


9/13

33

VoltageFollower

Realisticallythesecircuits

are

designedusingequalresis

tors(R1=

Rf)toavoidproblemswithoffset

voltagesandusefulasan

intermediate-stage(buffer)

amplifiertoisolateonecircuitfrom

another.Alsominimizesin

teraction

betweenthe2stagesandeliminates

interstageloading.

Anyamplifierwithnogainorlossiscalledaunitygainamplifier.

Theadvantagesofusingaunitygainamplifier:

Veryhighinputim

pedance

Verylowoutputim

pedance

34

SummingAmplifier

Becausethe

op-amphasa

highinputim

pedance,the

multipleinputsare

treatedasseparateinputs.

++++

++++

====

3

3f

2

2f

1

1f

o

V

RR

V

RR

V

RR

V

35

Integrator

Theoutputistheintegral

oftheinput.Integratio

n

istheoperationof

summingtheareaunder

awaveformorcurveover

aperiodoftime.This

circuitisusefulinlow-

passfiltercircuitsand

sensorconditioning

circuits.

====

(t)

dt

v

RC1

(t)

v

1

o

36

Differentiator

Thedifferentiator

takesthede

rivativeof

theinput.T

hiscircuit

isusefulinhigh-pass

filtercircuits.

dt(

t)

dv

R

C

(t)

v

1

o

====


10/13

37

InstrumentationA

mplifiers

Input1

R3

+

+

R1

R5

+

R2

A1

A2

A3

Output

R4

R6

Gainset

Gainset

Input2

Thegainissetbya

singleresistorthatis

suppliedbytheuser.

Vin1

+Vcm R

G

Vin2

+V

cm

Vout=Acl(Vin2V

in1)

Theoutputvoltageisthe

closedloopgainsetby

RGmultipliedbythe

voltagedifferenceinthe

inputs.

Acl=1+2R/Rg

Someapplicationsrequ

iringtheamplificationofverylow-levelsignals,a

specialtypeofamplifierknownasaninstrumentationamplifierisused.

Thefirstadvantageisd

ifferentialinputimpedanceismuchhigher.

Commonmoderejectio

ncapabilityismuchbetter.

Aninstrumentationam

plifier(IA)amplifiesthevoltagedifferencebetween

itsterminals.Itisoptimizedforsmalldifferentialsignalsthatmayberiding

onalargecommonmodevoltages.

38

Instrum

entationAmplifiers

ThebandwidthofanyIA(orop-ampforthatmatter)is

lowerfor

highergain.ThegraphshowstheBWforvariousgainsforthe

AD622.

WhatistheBWforagainof

35?

0

Voltagegain

100

Frequency(H

z)

1

1k

10k

100k

1M

10M

10

100

1000

Readingthegraph,theBW

isapproximately200kHz.

39

InstrumentationAmp

lifiersGuarding

GuardingisavailableinsomeIAstoreducenoiseeffects.By

drivingtheshieldwiththecommon-modesignal,effectsofstray

capacitanceareeffe

ctivelycancelled.

Guardingisusefulin

applicationssuchas

transducer

interfacing,and

microphone

preampswherevery

smallsignalsneed

tobetransmitted.

40

Example

Whatistheran

geofoutputvoltageinthe

circuitiftheinp

utcanvaryfrom0.1Vto0.5V?

Solution


11/13

41

Example

Whatinputmustbeapplie

dtotheinputofthecircuit

toresultinanoutputof2.4V?

Solution

42

Example

Whatrangeof

outputvoltageisdeveloped

inthecircuit?

Solution

43

Example

Designafirstorderlowpa

ssfilterthathasagainof26dBandabandwidth

of

500Hz.Inyourdesign,letR1||R2

andchooseCtomeettheBWrequirem

ent.

Solution

44

Modulation

Modulationistheprocessofvaryingoneormorepropertiesofa

periodicwaveform,calledthecarriersignal,withamodu

lating

signalwhich

typicallycontainsinformationtobetransmit

ted.

Thisisdone

inasimilarfashiontoamusicianmodulatingatone

(aperiodicw

aveform)fromamusicalinstrumentbyvaryingits

volume,timingandpitch.

Thethreekeyparametersofaperiodicwaveformareits

amplitude("

volume"),itsphase("timing")anditsfrequen

cy

("pitch").

Anyofthese

propertiescanbemodifiedin

accordance

withalowfrequencysignalto

obtainthem

odulatedsignal.Typicallya

high-frequencysinusoid

waveformisused

ascarriersignal,butasquarewavepulse

trainmayalsobeused.


12/13

45

Linearization

Mostsensorsdono

tproduceoutputsinlinearrelationwiththe

input.

Thenon-linearresp

onseofsensorscanbecorrectedby

propersignalconditioningtechnique.

Linearizationisnec

essarywhensensorsproducevoltage

signalsthatarenot

linearlyrelatedtothephysical

measurement.

Linearizationisthe

processofinterpretingthesignalfromth

e

sensorandcanbe

doneeitherwithsignalconditioningor

throughsoftware.

46

Linearization

Thetransferfunctionformanyelectronicdevices,which

relatesthe

inputtooutput,containsanonlinearfac

tor.

Inmostcasesthisfactorissmallenoughtobe

ignored.

However,

insomeapplicationsitmustbecomp

ensated

eitherinhardwareorsoftware.

Thermoco

uplesaretheclassicexampleofasen

sorthat

requireslinearization.Theyhaveanonlinearrela

tionship

from

inputtemperaturetooutputvoltage,severe

enough

torequire

compensation.

Eg,byva

ryingthegainofanamplifierasafun

ctionof

inputlevelorbyvaryingthecurrentinfeedbacklo

opofan

amplifierinnonlinearrelationwithinputvoltage,certain

sensoroutputcanbelinearized.

47

DigitalSignalConditioning

Animportantapplicationareaofmodemcomputer

systemsistha

tofdigitalsignalprocessing.This

discipline

is

concerned

with

the

analysis

or

modificationo

f

digitally

represented

signals,

through

the

use

of

simple

mathematical

operations

48

Multiple

xer:veryusefulindataacquisitionsystem

whereonlyoneADCusedforfewanalogs

ignals.

Sample

&Hold:voltage-memorycircuitfo

rADC

process.

AnalogtoDigitalConverter(ADC):con

vertthe

analogs

ignalintodigitalform

D

igitalSignalConditioning

Elements


13/13

49

Multiplexer

Forageneraldataacq

uisitionsystemitsschematicblockdiagr

am

isasshownbelow.

50

Multiplexer

Severalan

alogsignalsareprocessedsequentiallythro

ugha

multiplexer,whichisadigitallycontrolledswitch.

Themultip

lexeracceptsparallelinputsfromseveralch

annelsand

provideso

neanalogoutputatatimeforconversiontodigitalform.

Theindividualanalogsignals

areapplied

directlyorafteramplification

ofsignal

conditioningtothemultiplexe

r.

Thesearefurtherconverted

todigitalsignalsbyADC.

51

Signalconditioningisanimportantcomponent

of

anycompletem

easurementsystem.

Regardlesswh

ichtypeofsensorsused,sign

al

conditioning

can

improve

the

accuracy,

effectiveness,a

ndsafetyofmeasurements.

Signalconditioninghasthecapabilitiessuchas

amplifications,isolation,andfiltering.

There

aretwo

types

ofsignalconditionin

g;

analoganddigitalsignalconditionings.

Summary

chapter_3_-_signal_conditioning.pdf

Documents