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app_eng

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2012-07-26 0人阅读 0 0 0 暂无简介 举报

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iithAnniversaryReaderBonusAskTheApplicationsEngineerANALOGDEVICESTECHNICALREFERENCEBOOKSPublishedbyAnalogDevicesNonlinearCircuitsHandbookTransducerInterfacingHandbookMixedSignalDesignSeminarNotesAmplifierApplicationsGuideSystemApplicationsGuideLinearDesignSeminarNotesPracticalAnalogDesignTechniquesHighSpeedDesignSeminarNotesADSPFamilyApplicationsHandbookPublishedbyPrenticeHall(availablefromAnalogDevices)AnalogDigitalConversionHandbookDigitalSignalProcessinginVLSIDSPApplicationsUsingtheADSPFamily(Vols)AskTheApplicationsEngineer©AnalogDevices,Inc,AllrightsreservedWORLDWIDEHEADQUARTERSOneTechnologyWayPOBox,Norwood,MA,USATel:(),Fax:()PrintedintheUnitedStatesofAmericaInformationfurnishedbyAnalogDevices,Inc,isbelievedtobeaccurateandreliableHowever,noresponsibilityisassumedbyAnalogDevices,Inc,foritsuse,norforanyinfringementsofpatentsorotherrightsofthirdpartieswhichmayresultfromitsuseNolicenseisgrantedbyimplicationorotherwiseunderanypatentorpatentrightsofAnalogDevicesUseinlifesupportapplications:ProductssoldbyAnalogDevicesarenotdesignedforuseinlifesupportequipmentwheremalfunctionofsuchproductscanreasonablybeexpectedtoresultinpersonalinjuryordeathBuyerusesorsellssuchproductsforlifesupportapplicationatbuyer’sownriskandagreestodefend,indemnify,andholdharmlessAnalogDevicesfromanyandalldamages,claims,suits,orexpenseresultingfromsuchuseAllbrandorproductnamesmentionedaretrademarksorregisteredtrademarksoftheirrespectiveholdersSpecificationsaresubjecttochangewithoutnoticeAskTheApplicationsEngineerthAnniversaryReaderBonusiiiTABLEOFCONTENTSPagePrefaceiv␣Multitroubles,Troublefromthestart,AboutlogcompensationresistorsJamesBryant␣␣␣WhenitcomestotrimmingJamesBryant␣␣␣VFconvertersJamesBryant␣␣␣Usedcorrectly,highspeedcomparatorsprovidemanyusefulcircuitfunctionsJohnSylvan␣␣␣OpampissuesJamesBryant␣␣␣OpampissuesNoiseJamesBryant␣␣OpampissuesNoise(continued),HowgrainyiscurrentflowJamesBryant␣␣SeminarsandSupportChrisHyde␣Supplydecoupling,nonidealitiesofresistorsJamesBryant␣PCboardtracksandgroundplanesJamesBryant␣VoltagereferencesJamesBryant␣Grounding(again)WaltKester␣Timereferences,MoreonTrimmingJamesBryant␣ConfusedaboutamplifierdistortionspecsWaltKester␣HighfrequencysignalcontaminationJamesBryant,withHermanGelbach␣AReaderNotes(reHighfrequencysignalcontamination)LeroyCordill␣Usingsigmadeltaconverters,partIOliJosefsson␣Usingsigmadeltaconverters,partII,andquestionsonnoiseinconvertersOliJosefsson␣Musta“bit”converterbebitmonotonicandsettletoppmDaveRobertsonandSteveRuscak␣SettlingtimePeterCheckovich␣Interfacingtoserialconverters,partIEamonNash␣Interfacingtoserialconverters,partIIEamonNash␣CapacitanceandcapacitorsSteveGuinta␣Currentfeedbackamplifiers,partIErikBarnes␣Currentfeedbackamplifiers,partIIErikBarnes␣IndexvivthAnniversaryReaderBonusAskTheApplicationsEngineerPREFACEANDINTRODUCTIONThisspecialissueofAnalogDialogueisofferedasabonustoourfaithfulreadersandanencouragementtonewreadersWehavereprintedherethepopularseries,entitled“AskTheApplicationsEngineer”,fromitsinceptioninthroughNumber*inWearecelebratingAnalogDialogue’sthsequentialyearinprint,justconcludedDuringthoseyears,AnalogDialoguehasinformed,enlightened,andoccasionallyeducatedmorethanagenerationofengineers,scientists,andtechniciansaboutAnalogDevices’sproducts,practice,andideasforhighperformancesignalprocessingbyanalog,digital,andmixedmeansThisthanniversarySelection,AskTheApplicationsEngineer,joinsthecollectionofarticlesinourthanniversarybonusedition,TheBestofAnalogDialoguetoReadersofthatpagecollection(stillavailableuponrequest)willrecallitsmanyarticlesrelatingtopracticeaswellasarticlesaboutlandmarkproductsOurintentinoriginatingthe“AskTheApplicationsEngineer”serieshasbeentosupplementthatmineofinformationwithasourceofwhathascometobecalledFAQfrequentlyaskedquestions(andanswers)TheyarechronicledbyafewofourApplicationsEngineers,butarebasedontheaccumulatedwisdomandexperienceofourworldwideApplicationsEngineeringstaff,interactingwithtensofthousandsofcustomersworldwideThroughoutthepagesofthisvolumeyouwillfindreferencestoAnalogDevicesproductsandpublications,justastheyappearedintheoriginaleditionIfyouareinterestedinreceivingcopiesofpublicationsavailablefromAnalogDevicesorinformationaboutspecificproducts,youmayconsultourWebsite,http:wwwanalogcom,orphoneourHelpline,ANALOGD()inNorthAmerica,()␣elsewhere,oroneofourlocalofficesDatasheetsareavailableinNorthAmericabyhourautomatedfaxbyphoningtheAnalogFax™number,Requestsforpublicationsmayalsobefaxedto()␣Naturally,anytimelymaterialisonitswaytoobsolescencethedayitappearsinprint,soit’spossiblethatatthetimeyoureadaboutaparticulardevice,updatedorimprovedversionshavebecomeavailableForupdatedinformation,phoneourHelplineoroneofourlocalofficesThearticlesinthispublicationappearinchronologicalorder,startingwith“AskTheApplicationsEngineer”Butinorderforthebooktobeusefulasareferencesource,wehaveprovidedanIndexatthebackitwillpermitthereadertofindmaterialrelatingtoaparticularinterestwhereveritappearsWehopeyouwillfindthispublicationhelpfulandwillwelcomeyoursuggestionsofquestionstobeansweredinthefutureDanSheingoldanalogcomEditor,AnalogDialogue*Withoneexception:Wehavenotincluded“AskTheApplicationsEngineer”,whichwasdevotedtoadescriptionofa"ComponentSelectionGuidediskettethatwasnovelandusefulinitstimehowever,ithaslongbeenobsoleteinthisdayoftheWebsite,theCDROM,andtheselectiontreeinprintJustabouteverythingelsehasturnedouttoincludeinformationoftimelessvalueAnalogDialogue()AskTheApplicationsEngineerbyJamesBryantMULTITROUBLESQMymultiplexedADCsystemismisbehavingABeforeyougoanyfurther,haveyougroundedallunusedmultiplexerchannelsQNoButhowdidyouknowABecausethefloatingterminalisoneofthecommonestcausesofproblemsinsystemscontainingCMOSmultiplexersUnusedMUXinputsandoutputs(whetherintegratedintoamultiplexedADCorpartofaselfcontainedMUXchip)canpickupsignalsfromstrayfieldsandinjectthemintothedevice’ssubstrate,turningonspurioussubstratedevicesThen,evenwhentheunusedchannelisturnedoff,theperformanceoftheonchannelmaybebadlydegraded(attheunlikelyextreme,theinjectionmayturnonaspuriousfourlayerdeviceanddestroysomechips)WheneveraMUXisused,allitsinputsandoutputsmustbeconnectedtoapotentialbetweenitssupplyrailsThebestwaytodealwithunusedchannelsistogroundthem,buttheymaybeconnectedtoamoreconvenientpotentialwithintherailsbTROUBLEFROMTHESTARTQTosavepower,myADCispowereduponlytomakeameasurementThesystemisveryaccurateincontinuousoperation,butunpredictablewhenpowerisstrobedWhyAWhenanADC’spowerisswitchedononlytoperformaconversion,itmaymisbehaveforthreereasons:slowreferenceturnon,randominitiallogicstates,andsystemlatchupForvariousreasonsthermalstabilization,capacitancecharging,slowstartingofregenerativecurrentmirrorsusingPNPtransistorsinbandgapreferencesitisnotuncommonforsomevoltagereferencestohaverelativelylargeerrorsformanymillisecondsafterpowerupSucherrorsinanADC’sexternalorinternalreferenceduringconversionleadtoinaccurateresultsAtturnon,atypicalADC’slogicwillbeinarandomstateforaconversiontriggeredatthattime,theADCmaynotbeabletoperformcorrectlyWithoneconversiontriggered,thelogicshouldreturntoitscorrectpreconversionstatebutcasesexistwheretwoconversioncyclesarenecessarybeforetheADCiscertaintoperformavalidconversionHence,agoodgeneralruleistoperformtwo“dummy”conversionsafterpowerupbeforerelyingontheresults(ItisalsowelltorecallthatsomeADCsreactbadlytohavingaconversiontriggeredbeforethepreviousconversioniscompletewhenthishappens,oneortwo“dummy”conversionsmaybeneededtoreturnthelogictoaknownstate)IfanADC’sexternallogicisarrangedsothattheendoftheADC“Busy”signalstartsadelaywhichendswiththestartofthenextconversion,itisimportanttorealizethatiftheconverterpowersupintheBusystate,theBusysignalmayremainlatchedupuntilaconversionStartpulsehasbeenreceivedInthiscase,suchasystemcannotselfstartIftheBusysignalisalwayspresentonpoweruptheproblemisalmostcertaintoberecognizedandaddressedduringthedesignofthesystembutiftheBusysignalisonlyoccasionallypresentonpowerupthesystemmaylatchunpredictablyAsarule,controlsignalstoanADCduringstartupshouldnotdependonthelogicalstateofBusybABOUTLOGCOMPENSATIONRESISTORSQDesignsoflogarithmiccircuits*,includingthoseusingtheADYZXmunit:(forexample,FigurefromtheADMultifunctionUnitdatasheet†)callfor“kTqcompensationresistors”WhataretheyandwheredoIgetthemATheVBEdifferenceacrosstwoopposedsiliconjunctions,onecarryingacurrent,I,andtheotheracurrent,IREF,is(kTq)ln(IIREF)Here,kqistheratioofBoltzmann’sconstanttothechargeonanelectron(about,KV),andTistheabsolutetemperatureinkelvinsAlthoughemployingsimilarjunctionsinisothermalpairseliminatestheeffectsoftemperaturesensitivityofreversesaturationcurrent,thekTqtermisstilltemperaturedependentToeliminatethisdependencyintheapplication,thelogarithmicvoltagemustbeusedinacircuitwhosegainisinverselyproportionaltotheabsolutetemperatureofthejunctionsOverareasonablerangeoftemperaturesnear°C,thismaybearrangedbytheuseofagainsettingkΩresistorhavingapositivetemperaturecoefficientofapproximately,ppm°CandkeepingitatthesametemperatureasthejunctionsA,ppm°CresistorisavailablefromTelLaboratories,GHarveyRoad,Londonderry,NewHampshire(),Telex:(),designatedQ,andfromthePrecisionResistorCoInc,,thSt,Largo,Florida()Telex:,asthePTAnalogDevicesofficesinmostEuropeancountriesareawareoflocalsuppliersoftheseresistorsb*MuchusefulinformationaboutlogarithmicandotheranalogfunctioncircuitscanbefoundintheNonlinearCircuitsHandbook,publishedbyAnalogDevices($),POBox,NorwoodMA†SeeTheBestofAnalogDialogue–,ppAskTheApplicationsEngineerbyJamesBryantWHENITCOMESTOTRIMMINGQIneedsomeadviceabouttrimmingoffsetsandgainsADon’t!unlessyoumustGoodalternativesinclude(a)usingheadachefreedevices,components,andcircuitsthatmeetthespecswithouttrimming(b)takingadvantageofdigitaltechnologyinsystemapplicationstomaketrimcorrectionsinsoftwareSavingsprovidedonoccasionbytrimpotentiometers,inconjunctionwithlooselyspec’ddevices,canturnouttobeillusorywhenyouconsidertheeffectsofcircuitdesign,temperature,vibration,andlifeonperformanceandstabilityaswellasadditionalpaperworkandcomplexitytrimmingentailsQNevertheless,howdoItrimtheoffsetandgainerrorsinanalogcircuitryAInthecorrectorderandwiththecorrectinputsIfyouconsiderthetransfercharacteristicofthecircuitbeingtrimmedthemethodtouseisgenerallystraightforwardThesimplifiedidealtransfercharacteristicofalinearanalogcircuit(suchasanamplifier,ADCorDAC)isgivenbytheequation:OP=K×IP()whereOPisoutput,IPisinput,andKisascalefactor(Notethatthissimplificationhidesanenormousnumberofissues:quantizationerrorinanADCdimensionalityofKiftheinputandoutputareindifferentformsegvoltageincurrentoutintentionaloffsetsandmanyothers)Inareal(nonideal)circuit,offsetandgainerrors,OS(referredtotheinput)and∆K,respectively,alsoappearintheequation,whichbecomes:OP=(K∆K)×(IPOS)()OP=(K×IP)(K×OS)(∆K×IP)(∆K×OS)()Equations()and()areincompleteinthattheyassumeonlyoneoffsetattheinputbutthisisthemostcommoncaseSystemswithseparateinputandoutputoffsetswillbeconsideredlaterFrom()weseethatitnotpossibletotrimgaindirectlywhenanunknownoffsetispresentOffsetmustbetrimmedfirstWithIPsetat,theoffsettrimisadjusteduntilOPisalsoGainmaythenbetrimmed:withaninputneartofullscale(FS),thegaintrimisadjustedtomaketheoutputobeyequation()QButwhataboutbipolarADCsandDACsAManyADCsandDACsmaybeswitchedbetweenunipolarandbipolaroperationsuchdevices,whereverpossible,shouldhavetheiroffsetandgaintrimmedintheunipolarmodeWhereitisnotpossible,orwheretheconverteristooperateonlyinthebipolarmode,otherconsiderationsapplyAbipolarconvertermaybeconsideredasaunipolarconverterwithalargeoffset(tobeprecise,anoffsetofMSBonehalfoffullscalerange)Dependingonthearchitectureused,thisbipolaroffset(BOS)mayormaynotbeaffectedbythegaintrimIfitissoaffected,equation()becomes:OP=K×(IP–BOS)()Inthiscaseoffsetistrimmedatanalogzero,afterwhichgainistrimmednearFSpositiveornegative,butusuallypositiveThisisnormallythemethodusedforDACswherethebipolaroffsetiswithintheDACIfthebipolaroffsetisnotaffectedbythegaintrim:OP=K×IP–BOS()HereoffsetistrimmedatFSnegativeandgainistrimmedat(orveryneartoseebelow)FSpositiveThismethodisusedformostADCsandforDACswherebipolaroffsetisobtainedbytheuseofopampsandresistorsexternaltotheDACNaturally,themethodsuggestedonthedatasheetshouldalwaysbefollowed,butwhereadatasheetisunobtainable,ingeneral,offsetshouldbetrimmedatanalogzeroforDACsandFSnegativeforADCsandnearFSpositiveforbothQWhydoyoukeepsaying“near”tofullscaleAAmplifiersandDACsmaybetrimmedatzeroandfullscaleInthecaseofaDAC,all’sthelargestdigitalinputpossibleshouldproduceanoutputLSBbelow“fullscale,”where“fullscale”isconsideredassomeconstanttimesthereferencethisfollowssincetheoutputofaDACisthenormalizedproductofthereferenceandthedigitalinputADCsarenottrimmedatzeroandFSTheoutputofanidealADCisquantized,andthefirstoutputtransition(fromto)takesplaceLSBabovethenominalvalueofall’sThereaftertransitionstakeplaceeveryLSBincreaseinanaloginputuntilthefinaltransitiontakesplaceLSBbelowFSAnonidealADCistrimmedbysettingitsinputtothenominalvalueofadesiredtransitionandthenadjustinguntiltheADCoutputflickersbetweenthetwovaluesequallyTheoffsetofanADCisthereforetrimmedwithaninputcorrespondingtothefirsttransition(ie,LSBabovezerooraboveFSnegativewhichis“near”zeroor“near”FSnegative)andthegainisthentrimmedatthelasttransition(ieLSBbelowFSpositivewhichis“near”FSpositive)ThisprocedureresultsinaninteractionbetweenthegainandoffseterrorsduringoffsettrimbutitshouldbetooslighttobesignificantQArethereanyotheranomaliesresultinginaneedtotrim“near”,ratherthanatfullscaleASynchronousvoltagetofrequencyconverters(SVFCs)areliabletoinjectionlockingphenomenawhentheiroutputfrequencyisharmonicallyrelatedtotheirclockfrequency,ie,whentheiroutputisverycloseto,orofclockfrequencyFSforanSVFCisclockfrequencyThepresenceofatrimtoolcanexacerbatetheproblemItisthereforeadvisabletotrimthegainofanSVFCataroundofFSAnalogDialogue()QWhataboutcircuitsrequiringboth“input”and“output”offsettrimACircuitssuchasinstrumentationandisolationamplifiersoftenhavetwostagesofdcgain,andthegainoftheinputstagescanbevariableThusatwostageamplifier,withaninpu

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