养殖技术资料12位ad574a转换器中英文翻译资料.docx

养殖技术资料12位ad574a转换器中英文翻译资料.docx

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养殖技术资料12位ad574a转换器中英文翻译资料

英文原文

12-BitA/DConverter

CIRCUITOPERATION

TheAD574Aisacomplete12-bitA/Dconverterwhichrequiresnoexternalcomponentstoprovidethecompletesuccessiveapproximationanalog-to-digitalconversionfunction.AblockdiagramoftheAD574AisshowninFigure1.

Figure1.BlockDiagramofAD574A12-BitA-to-DConverter

Whenthecontrolsectioniscommandedtoinitiateaconversion（asdescribedlater）,itenablestheclockandresetsthesuccessiveapproximationregister（SAR）toallzeros.Onceaconversioncyclehasbegun,itcannotbestoppedorrestartedanddataisnotavailablefromtheoutputbuffers.TheSAR,timedbytheclock,willsequencethroughtheconversioncycleandreturnanend-of-convertflagtothecontrolsection.Thecontrolsectionwillthendisabletheclock,bringtheoutputstatusflaglow,andenablecontrolfunctionstoallowdatareadfunctionsbyexternalcommand.

Duringtheconversioncycle,theinternal12-bitcurrentoutputDACissequencedbytheSARfromthemostsignificantbit（MSB）toleastsignificantbit（LSB）toprovideanoutputcurrentwhichaccuratelybalancestheinputsignalcurrentthroughthe5kΩ（or10kΩ）inputresistor.Thecomparatordetermineswhethertheadditionofeachsuccessively-weightedbitcurrentcausestheDACcurrentsumtobegreaterorlessthantheinputcurrent;ifthesumisless,thebitislefton;ifmore,thebitisturnedoff.Aftertestingallthebits,theSARcontainsa12-bitbinarycodewhichaccuratelyrepresentstheinputsignaltowithin1/2LSB.

Thetemperature-compensatedburiedZenerreferenceprovidestheprimaryvoltagereferencetotheDACandguaranteesexcellentstabilitywithbothtimeandtemperature.Thereferenceistrimmedto10.00volts0.2%;itcansupplyupto1.5mAtoanexternalloadinadditiontotherequirementsofthereferenceinputresistor（0.5mA）andbipolaroffsetresistor（1mA）whentheAD574Aispoweredfrom15Vsupplies.IftheAD574Aisusedwith12Vsupplies,orifexternalcurrentmustbesuppliedoverthefulltemperaturerange,anexternalbufferamplifierisrecommended.AnyexternalloadontheAD574Areferencemustremainconstantduringconversion.Thethin-filmapplicationresistorsaretrimmedtomatchthefull-scaleoutputcurrentoftheDAC.Therearetwo5kinputscalingresistorstoalloweithera10voltor20voltspan.The10kbipolaroffsetresistorisgroundedforunipolaroperationandconnectedtothe10voltreferenceforbipolaroperation.

DRIVINGTHEAD574ANALOGINPUT

Figure2.OpAmp–AD574AInterface

Theoutputimpedanceofanopamphasanopen-loopvaluewhich,inaclosedloop,isdividedbytheloopgainavailableatthefrequencyofinterest.Theamplifiershouldhaveacceptableloopgainat500kHzforusewiththeAD574A.Tocheckwhethertheoutputpropertiesofasignalsourcearesuitable,monitortheAD574’sinputwithanoscilloscopewhileaconversionisinprogress.Eachofthe12disturbancesshouldsubsideinsorless.

Forapplicationsinvolvingtheuseofasample-and-holdamplifier,theAD585isrecommended.TheAD711orAD544opampsarerecommendedfordcapplications.

SAMPLE-AND-HOLDAMPLIFIERS

AlthoughtheconversiontimeoftheAD574Aisamaximumof35s,toachieveaccurate12-bitconversionsoffrequenciesgreaterthanafewHzrequirestheuseofasample-and-holdamplifier（SHA）.IfthevoltageoftheanaloginputsignaldrivingtheAD574Achangesbymorethan1/2LSBoverthetimeintervalneededtomakeaconversion,thentheinputrequiresaSHA.

TheAD585isahighlinearitySHAcapableofdirectlydrivingtheanaloginputoftheAD574A.TheAD585’sfastacquisitiontime,lowapertureandlowaperturejitterareideallysuitedforhigh-speeddataacquisitionsystems.ConsidertheAD574Aconverterwitha35sconversiontimeandaninputsignalof10Vp-p:

themaximumfrequencywhichmaybeappliedtoachieveratedaccuracyis1.5Hz.However,withtheadditionofanAD585,asshowninFigure3,themaximumfrequencyincreasesto26kHz.

TheAD585’slowoutputimpedance,fast-loopresponse,andlowdroopmaintain12-bitsofaccuracyunderthechangingloadconditionsthatoccurduringaconversion,makingitsuitableforuseinhighaccuracyconversionsystems.ManyotherSHAscannotachieve12-bitsofaccuracyandcanthuscompromiseasystem.TheAD585isrecommendedforAD574Aapplicationsrequiringasampleandhold.

Figure3.AD574AwithAD585SampleandHold

SUPPLYDECOUPLINGANDLAYOUT

CONSIDERATIONS

ItiscriticallyimportantthattheAD574Apowersuppliesbefiltered,wellregulated,andfreefromhighfrequencynoise.Useofnoisysupplieswillcauseunstableoutputcodes.Switchingpowersuppliesarenotrecommendedforcircuitsattemptingtoachieve12-bitaccuracyunlessgreatcareisusedinfilteringanyswitchingspikespresentintheoutput.Rememberthatafewmillivoltsofnoiserepresentsseveralcountsoferrorina12-bitADC.

CircuitlayoutshouldattempttolocatetheAD574A,associatedanaloginputcircuitry,andinterconnectionsasfaraspossiblefromlogiccircuitry.Forthisreason,theuseofwire-wrapcircuitconstructionisnotrecommended.Carefulprintedcircuitconstructionispreferred.

UNIPOLARRANGECONNECTIONSFORTHEAD574A

TheAD574Acontainsalltheactivecomponentsrequiredtoperformacomplete12-bitA/Dconversion.Thus,formostsituations,allthatisnecessaryisconnectionofthepowersupplies（+5V,+12V/+15Vand–12V/–15V）,theanaloginput,andtheconversioninitiationcommand,asdiscussedonthenextpage.Analoginputconnectionsandcalibrationareeasilyaccomplished;theunipolaroperatingmodeisshowninFigure4.

Figure4.UnipolarInputConnections

Allofthethin-filmapplicationresistorsoftheAD574Aaretrimmedforabsolutecalibration.Therefore,inmanyapplications,nocalibrationtrimmingwillberequired.Theabsoluteaccuracyforeachgradeisgiveninthespecificationtables.Forexample,ifnotrimsareused,theAD574AKguarantees1LSBmaxzerooffseterrorand0.25%（10LSB）maxfull-scaleerror.（Typicalfull-scaleerroris2LSB.）Iftheoffsettrimisnotrequired,Pin12canbeconnecteddirectlytoPin9;thetworesistorsandtrimmerforPin12arethennotneeded.Ifthefull-scaletrimisnotneeded,a501%metalfilmresistorshouldbeconnectedbetweenPin8andPin10.

TheanaloginputisconnectedbetweenPin13andPin9fora0Vto+10Vinputrange,between14andPin9fora0Vto+20Vinputrange.TheAD574Aeasilyaccommodatesaninputsignalbeyondthesupplies.Forthe10voltspaninput,theLSBhasanominalvalueof2.44mV;forthe20voltspan,4.88mV.

Ifa10.24Vrangeisdesired（nominal2.5mV/bit）,thegaintrimmer（R2）shouldbereplacedbya50Ωesistor,anda200ΩtrimmerinsertedinserieswiththeanaloginputtoPin13forafull-scalerangeof20.48V（5mV/bit）,usea500trimmerintoPin14.Thegaintrimdescribedbelowisnowdonewiththesetrimmers.ThenominalinputimpedanceintoPin13is5kΩ,and10kΩintoPin14.

UNIPOLARCALIBRATION

TheAD574Aisintendedtohaveanominal1/2LSBoffsetsothattheexactanaloginputforagivencodewillbeinthemiddleofthatcode（halfwaybetweenthetransitionstothecodesaboveandbelowit）.Thus,thefirsttransition（from000000000000to000000000001）willoccurforaninputlevelof+1/2LSB（1.22mVfor10Vrange）.

IfPin12isconnectedtoPin9,theunitwillbehaveinthismanner,withinspecifications.Iftheoffsettrim（R1）isused,itshouldbetrimmedasabove,althoughadifferentoffsetcanbesetforaparticularsystemrequirement.Thiscircuitwillgiveapproximately15mVofoffsettrimrange.

Thefull-scaletrimisdonebyapplyingasignal1/2LSBbelowthenominalfullscale（9.9963fora10Vrange）.TrimR2togivethelasttransition（111111111110to111111111111）.

BIPOLAROPERATION

TheconnectionsforbipolarrangesareshowninFigure5.Again,asfortheunipolarranges,iftheoffsetandgainspecificationsaresufficient,oneorbothofthetrimmersshowncanbereplacedbya501%fixedresistor.Bipolarcalibrationissimilartounipolarcalibration.

Figure5.BipolarInputConnections

CONTROLLOGIC

TheAD574Acontainson-chiplogictoprovideconversioninitiationanddatareadoperationsfromsignalscommonlyavailableinmicroprocessorsystems.Figure6showstheinternallogiccircuitryoftheAD574A.

ThecontrolsignalsCE,CS,andR/Ccontroltheoperationoftheconverter.ThestateofR/CwhenCEandCSarebothasserteddetermineswhetheradataread（R/C=1）oraconvert（R/C=0）isinprogress.TheregistercontrolinputsAOand12/8controlconversionlengthanddataformat.TheAOlineisusuallytiedtotheleastsignificantbitoftheaddressbus.IfaconversionisstartedwithAOlow,afull12-bitconversioncycleisinitiated.IfAOishighduringaconvertstart,ashorter8-bitconversioncycleresults.Duringdatareadoperations,AOdetermineswhetherthethree-statebufferscontainingthe8MSBsoftheconversionresult（AO=0）orthe4LSBs（AO=1）areenabled.The12/8pindetermineswhethertheoutputdataistobeorganizedastwo8-bitwords（12/8tiedtoDIGITALCOMMON）orasingle12-bitword（12/8tiedtoVLOGIC）.The12/8pinisnotTTL-compatibleandmustbehard-wiredtoeitherVLOGICorDIGITALCOMMON.Inthe8-bitmode,thebyteaddressedwhenAOishighcontainsthe4LSBsfromtheconversionfollowedbyfourtrailingzeroes.Thisorganizationallowsthedatalinestobeoverlappedfordirectinterfaceto8-bitbuseswithouttheneedforexternalthree-statebuffers.ItisnotrecommendedthatAOchangestateduringadatareadoperation.Asymmetricalenableanddisableti