AD736外文翻译电子教案.docx
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AD736外文翻译电子教案.docx
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AD736外文翻译电子教案
FEATURES
Computes
Truermsvalue
Averagerectifiedvalue
Absolutevalue
Provides200mVfull-scaleinputrange(largerinputswithinputattenuator)
Highinputimpedance:
1012Ω
Lowinputbiascurrent:
25pAmaximum
Highaccuracy:
±0.3mV±0.3%ofreading
RMSconversionwithsignalcrestfactorsupto5
Widepowersupplyrange:
+2.8V,−3.2Vto±16.5V
Lowpower:
200mAmaximumsupplycurrent
Bufferedvoltageoutput
Noexternaltrimsneededforspecifiedaccuracy
AD737—anunbufferedvoltageoutputversionwithchippower-downalsoavailable
GENERALDESCRIPTION
TheAD736isalowpower,precision,monolithictruerms-to-dcconverter.Itislasertrimmedtoprovideamaximumerrorof±0.3mV±0.3%ofreadingwithsinewaveinputs.Furthermore,itmaintainshighaccuracywhilemeasuringawiderangeofinputwaveforms,includingvariableduty-cyclepulsesandtriac(phase)-controlledsinewaves.Thelowcostandsmallsizeofthisconvertermakeitsuitableforupgradingtheperformanceofnon-rmsprecisionrectifiersinmanyapplications.Comparedtothesecircuits,theAD736offershigheraccuracyatanequalorlowercost.
TheAD736cancomputethermsvalueofbothacanddcinputvoltages.Itcanalsobeoperatedasanac-coupleddevicebyaddingoneexternalcapacitor.Inthismode,theAD736canresolveinputsignallevelsof100μVrmsorless,despitevariationsintemperatureorsupplyvoltage.Highaccuracyisalsomaintainedforinputwaveformswithcrestfactorsof1to3.Inaddition,crestfactorsashighas5canbemeasured(introducingonly2.5%additionalerror)atthe200mVfull-scaleinputlevel.
TheAD736hasitsownoutputbufferamplifier,therebypro-vidingagreatdealofdesignflexibility.Requiringonly200μAofpowersupplycurrent,theAD736isoptimizedforuseinportablemultimetersandotherbattery-poweredapplications.
TheAD736allowsthechoiceoftwosignalinputterminals:
ahighimpedanceFETinput(1012Ω)thatdirectlyinterfaceswithHigh-Zinputattenuatorsandalowimpedanceinput(8kΩ)thatallowsthemeasurementof300mVinputlevelswhileoperatingfromtheminimumpowersupplyvoltageof+2.8V,−3.2V.Thetwoinputscanbeusedeithersingleendedordifferentially.
TheAD736hasa1%readingerrorbandwidththatexceeds10kHzfortheinputamplitudesfrom20mVrmsto200mVrmswhileconsumingonly1mW.
TheAD736isavailableinfourperformancegrades.TheAD736JandAD736Kgradesareratedoverthe0°Cto+70°Cand−20°Cto+85°Ccommercialtemperatureranges.
TheAD736AandAD736Bgradesareratedoverthe−40°Cto+85°Cindustrialtemperaturerange.TheAD736isavailableinthreelowcost,8-leadpackages:
PDIP,SOIC,andCERDIP.
PRODUCTHIGHLIGHTS
1.TheAD736iscapableofcomputingtheaveragerectifiedvalue,absolutevalue,ortruermsvalueofvariousinputsignals.
2.Onlyoneexternalcomponent,anaveragingcapacitor,isrequiredfortheAD736toperformtruermsmeasurement.
3.Thelowpowerconsumptionof1mWmakestheAD736suitableformanybattery-poweredapplications.
4.Ahighinputimpedanceof1012Ωeliminatestheneedforanexternalbufferwheninterfacingwithinputattenuators.
5.Alowimpedanceinputisavailableforthoseapplicationsthatrequireaninputsignalupto300mVrmsoperatingfromlowpowersupplyvoltages.
SPECIFICATIONS
At25°C±5Vsupplies,ac-coupledwith1kHzsinewaveinputapplied,unlessotherwisenoted.Specificationsinboldaretestedonallproductionunitsatfinalelectricaltest.Resultsfromthosetestsareusedtocalculateoutgoingqualitylevels.
StressesabovethoselistedunderAbsoluteMaximumRatingsmaycausepermanentdamagetothedevice.Thisisastressratingonly;functionaloperationofthedeviceattheseoranyotherconditionsabovethoseindicatedintheoperationalsectionofthisspecificationisnotimplied.Exposuretoabsolutemaximumratingconditionsforextendedperiodsmayaffectdevicereliability.
THEORYOFOPERATION
AsshownbyFigure18,theAD736hasfivefunctionalsubsections:
theinputamplifier,full-waverectifier(FWR),rmscore,outputamplifier,andbiassection.TheFETinputamplifierallowsbothahighimpedance,bufferedinput(Pin2)andalowimpedance,widedynamicrangeinput(Pin1).Thehighimpedanceinput,withitslowinputbiascurrent,iswellsuitedforusewithhighimpedanceinputattenuators.
Theoutputoftheinputamplifierdrivesafull-waveprecisionrectifierthat,inturn,drivesthermscore.Theessentialrmsoperationsofsquaring,averaging,andsquarerootingareperformedinthecoreusinganexternalaveragingcapacitor,CAV.WithoutCAV,therectifiedinputsignaltravelsthroughthecoreunprocessed,asisdonewiththeaveragerespondingconnection(seeFigure19).
Afinalsubsection,anoutputamplifier,bufferstheoutputfromthecoreandallowsoptionallow-passfilteringtobeperformedviatheexternalcapacitor,CF,whichisconnectedacrossthefeedbackpathoftheamplifier.Intheaveragerespondingconnection,thisiswherealloftheaveragingiscarriedout.Inthermscircuit,thisadditionalfilteringstagehelpsreduceanyoutputripplethatwasnotremovedbytheaveragingcapacitor,CAV.
TYPESOFACMEASUREMENT
TheAD736iscapableofmeasuringacsignalsbyoperatingaseitheranaveragerespondingconverteroratruerms-to-dcconverter.Asitsnameimplies,anaveragerespondingconvertercomputestheaverageabsolutevalueofanac(oracanddc)voltageorcurrentbyfull-waverectifyingandlow-passfilteringtheinputsignal;thisapproximatestheaverage.Theresultingoutput,adcaveragelevel,isscaledbyadding(orreducing)gain;thisscalefactorconvertsthedcaveragereadingtoanrmsequivalentvalueforthewaveformbeingmeasured.Forexample,theaverageabsolutevalueofasinewavevoltageis0.636timesVPEAK;thecorrespondingrmsvalueis0.707×VPEAK.Therefore,forsinewavevoltages,therequiredscalefactoris1.11(0.707/0.636).
Incontrasttomeasuringtheaveragevalue,truermsmeasurementisauniversallanguageamongwaveforms,allowingthemagnitudesofalltypesofvoltage(orcurrent)waveformstobecomparedtooneanotherandtodc.RMSisadirectmeasureofthepowerorheatingvalueofanacvoltagecomparedtothatofadcvoltage;anacsignalof1Vrmsproducesthesameamountofheatinaresistorasa1Vdcsignal.
Mathematically,thermsvalueofavoltageisdefined(usingasimplifiedequation)as
Thisinvolvessquaringthesignal,takingtheaverage,andthenobtainingthesquareroot.Truermsconvertersaresmartrectifiers;theyprovideanaccuratermsreadingregardlessofthetypeofwaveformbeingmeasured.However,averagerespondingconverterscanexhibitveryhigherrorswhentheirinputsignalsdeviatefromtheirprecalibratedwaveform;themagnitudeoftheerrordependsonthetypeofwaveformbeingmeasured.Forexample,ifanaveragerespondingconverteriscalibratedtomeasurethermsvalueofsinewavevoltagesandthenisusedtomeasureeithersymmetricalsquarewavesordcvoltages,theconverterhasacomputationalerror11%(ofreading)higherthanthetruermsvalue(seeTable4).
CALCULATINGSETTLINGTIMEUSINGFIGURE16
Figure16canbeusedtocloselyapproximatethetimerequiredfortheAD736tosettlewhenitsinputlevelisreducedinamplitude.Thenettimerequiredforthermsconvertertosettleisthedifferencebetweentwotimesextractedfromthegraph(theinitialtimeminusthefinalsettlingtime).Asanexample,considerthefollowingconditions:
a33μFaveragingcapacitor,a100mVinitialrmsinputlevel,andafinal(reduced)1mVinputlevel.FromFigure16,theinitialsettlingtime(wherethe100mVlineintersectsthe33μFline)isapproximately80ms.
Thesettlingtimecorrespondingtotheneworfinalinputlevelof1mVisapproximately8seconds.Therefore,thenettimeforthecircuittosettletoitsnewvalueis8secondsminus80ms,whichis7.92seconds.Notethatbecauseofthesmoothdecaycharacteristicinherentwithacapacitor/diodecombination,thisisthetotalsettlingtimetothefinalvalue(thatis,notthesettlingtimeto1%,0.1%,andsoon,ofthefinalvalue).Inaddition,thisgraphprovidestheworst-casesettlingtimebecausetheAD736settlesveryquicklywithincreasinginputlevels.
RMSMEASUREMENT—CHOOSINGTHEOPTIMUMVALUEFORCAV
Becausetheexternalaveragingcapacitor,CAV,holdstherectifiedinputsignalduringrmscomputation,itsvaluedirectlyaffectstheaccuracyofthermsmeasurement,especiallyatlowfrequencies.Furthermore,becausetheaveragingcapacitorappearsacrossadiodeinthermscore,theaveragingtimeconstantincreasesexponentiallyastheinputsignalisreduced.Thismeansthatastheinputleveldecreases,errorsduetononidealaveragingdecrease,andthetimerequiredforthecircuittosettletothenewrmslevelincreases.Therefore,lowerinputlevelsallowthecircuittoperformbetter(duetoincreasedaveraging)butincreasethewaitingtimebetweenmeasurements.Obviously,whenselectingCAV,atrade-offbetweencomputationalaccuracyandsettlingtimeisrequired.
RAPIDSETTLINGTIMESVIATHEAVERAGERESPONDINGCONNECTION
BecausetheaveragerespondingconnectionshowninFigure19doesnotusetheCAVaveragingcapacitor,itssettlingtimedoesnotvarywiththeinputsignallevel.ItisdeterminedsolelybytheRCtimeconstantofCFandtheinternal8kΩresistorintheoutputamplifier’sfeedbackpath.
DCERROR,OUTPUTRIPPLE,ANDAVERAGINGERROR
Figure20showsthetypicaloutputwaveformoftheAD736withasinewaveinputapplied.Aswithallreal-worlddevices,theidealoutputofVOUT=VINis
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