AD736外文翻译电子教案.docx

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