传感器 第三章

CHAPTER3AMPLIFIERSFORSIGNALCONDITIONING1.INTRODUCTIONThissectionexaminesthecriticalparametersofamplifiersforuseinprecisionsignalconditioningapplications.OffsetvoltagesforprecisionICopampscanbelowas10uVwithcorrespondingtemperaturedriftsof0.1uV/oC.Openloopgainsgreaterthan1millionarecommon,alongwithcommonmodeandpowersupplyrejectionratiosofthesamemagnitude.Applyingtheseprecisionamplifierswhilemaintainingtheamplifierperformancecanpresentsignificantchallengestoadesignengineer,i.e.,externalpassivecomponentselection.ItisimportanttounderstandthatDCopen-loopgain,offsetvoltage,andcommonmoderejection(CMR)aloneshouldnotbetheonlyconsiderationsinselectingprecisionamplifiers.TheACperformanceoftheamplifierisalsoimportant,evenat"low"frequencies.Forexample,theopenloopgainis10at100kHz,and100,000at10Hz.Lossofopenloopgainatthefrequencyofinterestcanintroducedistortion,especiallyataudiofrequencies.LossofCMRatthelinefrequencyorharmonicsthereofcanalsointroduceerrors.Inaddition,awideselectionofprecisionamplifiersarenowavailablewhichoperateonsinglesupplyvoltageswhichcomplicatesthedesignprocessevenfurtherbecauseofthereducedsignalswingsandvoltageinputandoutputrestrictions.Offsetvoltageandnoisearenowamoresignificantportionoftheinputsignal.AMPLIFIERSFORSIGNALCONDITIONING~InputOffsetVoltage<100μV~InputOffsetVoltageDrift<1μV/oC~InputBiasCurrent<2nA~InputOffsetCurrent<2nA~DCOpenLoopGain>1,000,000~UnityGainBandwidthProduct,fu500kHz-5MHz~AlwaysCheckOpenLoopGainatSignalFrequency~1/f(0.1Hzto10Hz)Noise<1μVp-p~WidebandNoise<10nV/√Hz~CMR,PSR>100dB2.PRECISIONOPAMPCHARACTERISTICS2.1lnputOffsetVoltageInputoffsetvoltageerrorisusuallyoneofthelargesterrorsourcesforprecisionamplifiercircuitdesigns.However,itisasystemicerrorandcanusuallybedealtwithbyusingamanualoffsetnulltrimorbysystemcalibrationtechniquesusingamicrocontrollerormicroprocessor.Bothsolutionscarryacostpenalty,andtoday'sprecisionopampsofferinitialoffsetvoltagesaslowas10uVforbipolardevices,andfarlessforchopperstabilizedamplifiers.Measuringinputoffsetvoltagesofafewmicrovoltsrequiresthatthetestcircuitdosenotintroducemoreerrorthantheoffsetvoltageitself.���Themeasurementismadeattheamplifieroutputusingandigitalvoltmeter.Theoffsetreferredtotheinputiscalculatedbydividingtheoutputvoltagebythenoisegain.Assimpleasitlooks,thiscircuitmaygiveinaccurateresults.Thelargestpotentialsourceoferrorcomesfromparasiticthermocouplejunctionsformedwheretwodifferentmetalsarejoined.Thethermocouplevoltageformedbytemperaturedifferencebetweentwojunctionscanrangefrom2µV/oCtomorethan40µV/oC.Theaccuracyofthemeasurementdependonthemechanicallayoutofthecomponentsandhowtheyareplaced.Keepinmindthatthetwoconnectionsofacomponentsuchasaresistorcreatetwoequal,butoppositepolaritythermoelectricvoltageswhichcanceleachotherassumingbothareatexactlythesametemperature.OP177/AD707OFFSETADJUSTMENTPINSItisimportanttonotethattheoffsetdriftofanopampwithtemperaturewillvarywiththesettingofitsoffsetadjustment.Inmostcasesabipolaropampwillhaveminimumdriftatminimumoffset.Theoffsetadjustmentpinsshouldthereforebeusedonlytoadjusttheopamp'sownoffset,nottocorrectanysystemoffseterrorssincethiswouldbeattheexpenseofincreasedtemperaturedrift.��R1R21864-Vs+Vs7_+23ThedriftpenaltyforaJFETinputopampismuchworsethanforabipolarinputandisintheorderof4μV/oCforeachmillivoltofnulledoffsetvoltage.Itisgenerallybettertocontroltheoffsetvoltagebyproperselectionofdevicesanddevicegrades.Singleopampsinsmallpackagesdonotgenerallyhavenullcapability,andoffsetadjustmentsmustbedoneelsewherethesystemwhenusingthesedevices.2.2InputOffsetVoltageandInputBiasCurrentModelsThusfar,wehaveconsideredonlytheopampinputoffsetvoltage.However,inputbiascurrentsalsocontributetooffseterror.OPAMPTOTALOFFSETVOLTAGEMODEL�1�AC����BAR1R3IB+IB-R2VOS+-VOUTForaprecisionopamphavingastandardbipolarinputstageusingeitherPNPsorNPNs,theinputbiascurrentsaretypically50nAto400nAandarewellmatched.BymakingR3equaltotheparallelcombinationofR1andR2,theireffectonthenetRTIandRTOoffsetvoltageisapproximatelycanceled.2.3DCOpenLoopGainNonlinearityItiswellunderstoodthatinordertomaintainaccuracy,aprecisionamplifier’sDCopenloopgainAVOLshouldbehigh.Thiscanbeseenbyexaminingtheequationfortheclosedloopgain:IfAVOLintheaboveequationisinfinite,theclosedloopgainisexactlyequaltothenoisegain.However,forfinitevaluesofAVOL,thereisaclosedloopgainerrorgivenbytheequation:Noticefromtheequationthatthepercentgainerrorisdirectlyproportionaltothenoisegain,thereforetheeffectsoffiniteAVOLarelessforlowgain."IDEAL"CLOSEDLOOPGAIN=NOISEGAIN=NGACTUALCLOSEDLOOPGAIN=%CLOSEDLOOPGAINERROR=~AssumeAVOL=2,000,000,NG=1,000%GAINERROR≈0.05%~AssumeAVOLDropsto300,000%GAINERROR≈0.33%~CLOSEDLOOPGAINUNCERTAINTY=0.33%-0.05%=0.28%Changesintheoutputvoltagelevelandtheoutputloadingarethemostcommoncausesofchangesintheopenloopgainofopamps.Mostopampshavefixedloads,soopenloopgainchangeswithloadarenotgenerallyimportant.However,thesensitivityofopenloopgaintooutputsignallevelmayincreaseforhigherloadcurrents.Iftemperatureshiftisthesolecauseofthenonlinearityerror,itcanbeassumedthatminimizingtheoutputloadingwillhelp.Toverifythis,thenonlinearityismeasuredwithnoloadandthencomparedtotheloadedcondition.AnoscilloscopeX-YdisplaytestcircuitformeasuringDCopenloopgainnonlinearityisshownbelow.CIRCUITMEASURESOPENLOOPGAINNONLINEARITY2.4OpAmpNoiseThethreenoisesourcesinanopampcircuitarethevoltagenoiseoftheopamp,thecurrentnoiseoftheopamp(therearetwouncorrelatedsources,oneineachinput),andtheJohnsonnoiseoftheresistancesinthecircuit.Opampnoisehastwocomponents-"white"noiseatmediumfrequenciesandlowfrequency"1/f'noise,whosespectraldensityisinverselyproportionaltothesquarerootofthefrequency.INPUTVOLTAGENOISE,nV/Hz2.5CommonModeRejectionandPowerSupplyRejectionIfasignalisappliedequallytobothinputsofanopampsothatthedifferentialinputvoltageisunaffected,theoutputshouldnotbeaffected.Inpractice,changesincommonmodevoltagewillproducechangesintheoutput.ThecommonmoderejectionratioorCMRRistheratioofthecommonmodegaintothedifferentialmodegainofanopamp.Forexample,ifadifferentialinputchangeofYvoltswillproduceachangeof1Vattheoutput,andacommonmodechangeofXvoltsproducesasimilarchangeof1V,thentheCMRRisX/Y.itisnormallyexpressedindB,andtypicalLFvaluesarebetween70and120dB.WhenexpressedindB,itisgenerallyreferredtoascommonmoderejection(CMR).Athigherfrequencies,CMRdeteriorates.CMR=20log10CMRRCMRRproducesacorrespondingoutputoffsetvoltageerrorinopampsconfiguredinthenon-invertingmode.�1���+-VIN=VCMR1R2VOUTPSR=20log10PSRRIfthesupplyofanopampchanges,itsoutputshouldnot,butitwill.ThespecificationofpowersupplyrejectionratioorPSRRisdefinedsimilarlytothedefinitionofCMRR.IfachangeofXvoltsinthesupplyproducesthesameoutputchangeasadifferentialinputchangeofYvolts,thenthePSRRonthatsupplyisX/Y.WhentheratioisexpressedindB,itisgenerallyreferredtoaspowersupplyrejection,orPSR.ThedefinitionofPSRRassumesthatbothsuppliesarealteredequallyinoppositedirections-otherwisethechangewillintroduceacommonmodechangeaswellasasupplychange,andtheanalysisbecomesconsiderablymorecomplex.3.SINGLESUPPLYOPAMPSOverthelastseveralyears,single-supplyoperationhasbecomeanincreasinglyimportantrequirementbecauseofmarketrequirements.AccuracyandprecisionrequirementshaveforcedICmanufacturerstomeetthechallengeof"doingmorewithless"intheiramplifierdesigns.SINGLESUPPLYAMPLIFIERS~SingleSupplyOffers:~LowerPower~BatteryOperatedPortableEquipment~DesignTradeoffs:~ReducedSignalSwingIncreasesSensitivitytoErrorsCausedbyOffsetVoltage,BiasCurrent,FiniteOpen-LoopGain,Noise,etc.~MustUsuallyShareNoisyDigitalSupply~Rail-to-RailInputandOutputNeededtoIncreaseSignalSwing~PrecisionLessthanthebestDualSupplyOpAmpsbutnotRequiredforAllApplicationsInasingle-supplyapplication,themostimmediateeffectontheperformanceofanamplifieristhereducedinputandoutputsignalrange.Asaresultoftheselowerinputandoutputsignalexcursions,amplifiercircuitsbecomemoresensitivetointernalandexternalerrorsources.Tokeepbatterycurrentdrainlow,largerresistorsareusuallyusedaroundtheopamp.Sincethebiascurrentflowsthroughtheselargerresistors,theycangenerateoffseterrorsequaltoorgreaterthantheamplifier'sownoffsetvoltage.4.INSTRUMENTATIONAMPLIFIERS(IN-AMPS)Aninstrumentationamplifierisaclosed-loopgainblockwhichhasadifferentialinputandanoutputwhichissingle-endedwithrespecttoareferenceterminal.Inordertobeeffective,anin-ampneedstobeabletoamplifymicrovolt-levelsignals,whilesimultaneouslyrejectingvoltsofcommonmodesignalatitsinputs.Thisrequiresthatin-ampshaveveryhighcommonmoderejection(CMR):typicalvaluesofCMRare70dBtoover100dB,withCMRusuallyimprovingathighergains.SpecifyingCMRoverfrequencyismoreimportantthanspecifyingitsDCvalue.ItisimportanttonotethataCMRspecificationforDCinputsaloneisnotsufficientinmostpracticalapplications.4.1InstrumentationAmplifierConfigurationsLow-frequencyCMRofopamps,connectedassubtractorsasshownbelow.Amismatchofonly0.1%intheresistorratioswillreducetheDCCMRtoapproximately66dB.Anotherproblemwiththesimpleopampsubtractoristhattheinputimpedancesarerelativelylowandareunbalancedbetweenthetwosides.Thesimplesubtractorcircuitdescribedabovelackstheperformancerequiredforprecisionapplications.Anin-amparchitecturewhichovercomessomeoftheweaknessesofthesubtractorcircuitusestwoopamps.VREFThiscircuitistypicallyreferredtoasthetwoopampin-amp.DualICopampsareusedinmostcasesforgoodmatching.Theinputimpedanceishigh,permittingtheimpedanceofthesignalsourcestobehighandunbalanced.Thereisanimplicitadvantagetothisconfigurationduetothegainexecutedonthesignal.ThisraisestheCMRinproportion.Whilethinfilmresistorsfabricatedonsiliconhaveaninitialtoleranceofupto+20%,lasertrimmingduringproductionallowstheratioerrorbetweentheresistorstobereducedto0.01%(100ppm).Furthermore,thetrackingbetweenthetemperaturecoefficientsofthethinfilmresistorsisinherentlylowandistypicallylessthan3ppm/oC(0.0003%/oC).Whendualsuppliesareused,VREFisnormallyconnecteddirectlytoground.Insinglesupplyapplications,VREFisusuallyconnectedtoalowimpedancevoltagesourceequaltoone-halfthesupplyvoltage.ThegainfromVREFtonode"A"isR1/R2,andthegainfromnode"A"totheoutputisR2'/R1'.Onemajordisadvantageofthisdesignisthatcommonmodevoltageinputrangemustbetradedoffagainstgain.TheamplifierA1mustamplifythesignalatV1by’1+R1/R2’.IfR1>>R2,A1willsaturateifthecommonmodesignalistoohigh,leavingnoheadroomtoamplifythewanteddifferentialsignal.Forhighgains(R1<<R2),thereiscorrespondinglymoreheadroomatnode"A"allowinglargercommonmodeinputvoltages.TheACcommonmoderejectionofthisconfigurationisgenerallypoorbecausethesignalfromV1toVOUThastheadditionalphaseshiftofA1.Inaddition,thetwoamplifiersareoperatingatdifferentclosed-loopgains(andthusatdifferentbandwidths).Theuseofasmalltrimcapacitor"C"asshowninthediagramcanimprovetheACCMRsomewhat.Alowgain(G=2)singlesupplytwoopampin-ampconfigurationresultswhenRGisnotused.TheinputcommonmodeanddifferentialsignalsmustbelimitedtovalueswhichpreventsaturationofeitherA1orA2.TheirupperandloweroutputlimitsaredesignatedVOHandVOL,respectively.Usingtheequationsshowninthediagram,thevoltageatV1mustfallbetween1.3Vand3.7VtopreventA1fromsaturating.NoticethatVREFisconnectedtotheaverageofVOHandVOL(2.5V).ThisallowsforbipolardifferentialinputsignalswithVOUTreferencedto+2.5V.VREF=SINGLESUPPLYRESTRICTIONS:VS=+5V,G=2V2V1SINGLESUPPLYRESTRICTIONS:Vs=+5V,G=100              VREF=NotethatthevoltageatV1cannowswingbetween0.124Vand4.876V.V2V1Theabovediscussionshowsthatregardlessofgain,thebasictwoopampin-ampdoesnotallowforzero-voltcommonmodeinputvoltageswhenoperatedonasinglesupply.ThislimitationcanbeovercomeusingthecircuitshownbelowwhichisimplementedintheAD627in-amp.ThePNPtransistorsnotonlyprovidegainbutalsolevelshifttheinputsignalpositivebyabout0.5V,therebyallowingthecommonmodeinputvoltagetogoto0.1Vbelowthenegativesupplyrail.TheAD627in-ampdeliversrail-to-railoutputswingandoperatesoverawidesupplyvoltagerange(+2.7Vto18V).WithoutRG,theexternalgainsettingresistor,thein-ampgainis5.Gainsupto1000canbesetwithasingleexternalresistor.AD627IN-AMPKEYSPECIFICATIONS~WideSupplyRange:+2.7Vto18V~InputVoltageRange:-Vs-0.1Vto+Vs-1V~85µASupplyCurrent~GainRange:5to1000~75µVMaximumInputOffsetVoltage(AD627B)~10ppm/oCMaximumOffsetVoltageTC(AD627B)~10ppmGainNonlinearity~85dBCMR@60Hz,1kΩSourceImbalance(G=5)~3µVp-p0.1Hzto10HzInputVoltageNoise(G=5)Fortruebalancedhighimpedanceinputs,threeopampsmaybeconnectedtoformthein-ampshownbelow.Thiscircuitistypicallyreferredtoasthethreeopampin-amp.Thegainoftheamplifierissetbytheresistor,RG,whichmaybeinternal,external,or(softwareorpin-strap)programmable.Inthisconfiguration,CMRdependsupontheratiomatchingofR3/R2toR3'/R2'.TheclassicthreeopampconfigurationhasbeenusedinanumberofmonolithicICinstrumentationamplifiers.Besidesofferingexcellentmatchingbetweenthethreeinternalopamps,thinfilmlasertrimmedresistorsprovideexcellentratiomatchingandgainaccuracyatmuchlowercostthanusingdiscreteopampsandresistornetworksTheAD620isanexcellentexampleofmonolithicin-amptechnologyTheAD620isahighlypopularin-ampandisspecifiedforpowersupplyvoltagesfrom2.3Vto18V.ThegainissetwithasingleexternalRGresistor.4.2InstrumentationAmplifierDCErrorSourcesTheDCandnoisespecificationsforinstrumentationamplifiersdifferslightlyfromconventionalopamps,sosomediscussionisrequiredinordertofullyunderstandtheerrorsources.Thegainofanin-ampisusuallysetbyasingleresistor.Oftenspecifiedashavingagainrangeof1to1000,or1to10,000,manyin-ampswillworkathighergains,butthemanufacturerwillnotguaranteeaspecificlevelofperformanceatthesehighgains.Inpractice,asthegain-settingresistorbecomessmaller,anyerrorsduetotheresistanceofthemetalrunsandbondwiresbecomesignificant.Theseerrors,alongwithanincreaseinnoiseanddrift,maymakehighersingle-stagegainsimpractical.Inaddition,inputoffsetvoltagescanbecomequitesizablewhenreflectedtooutputathighgains.Inapin-programmablegainin-ampsuchastheAD621,thegainsettingresistorsareinternal,wellmatched,andthegainaccuracyandgaindriftspecificationsincludetheireffects.Thegainerrorspecificationisthemaximumdeviationfromthegainequation.Monolithicin-ampssuchastheAD624Chaveverylowfactorytrimmedgainerrors,withitsmaximumerrorof0.02%atG=1and0.25%atG=500beingtypicalforthishighqualityin-amp.4.3In-AmpBridgeAmplifierErrorBudgetAnalysisItisimportanttounderstandin-amperrorsourcesinatypicalapplication.TheFigureshowsa350Ωloadcellwhichhasafullscaleoutputof100mVwhenexcitedwitha10Vsource.TheAD620isconfiguredforagainof100usingtheexternal499Ωgain-settingresistor.