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FIGURE5.AccommodatingaHighSpeedSystem
2.0ANALOGCONSIDERATIONS
ThefundamentalpurposeofanyDtoAconverteristopro-videanaccurateanalogoutputquantitywhichisrepresenta-tiveoftheapplieddigitalword.InthecaseoftheDAC0830,theoutput,IOUT1,isacurrentdirectlyproportionaltotheproductoftheappliedreferencevoltageandthedigitalinputword.Forapplicationversatility,asecondoutput,IOUT2,isprovidedasacurrentdirectlyproportionaltothecomplementofthedigitalinput.Basically:
putlevel(“1”or“0”)respectively,asshowninFigure6.TheMOSswitchesoperateinthecurrentmodewithasmallvolt-agedropacrossthemandcanthereforeswitchcurrentsofeitherpolarity.Thisisthebasisforthe4-quadrantmultiplyingfeatureofthisDAC.
2.2BasicUnipolarOutputVoltage
Tomaintainlinearityofoutputcurrentwithchangesintheap-plieddigitalcode,itisimportantthatthevoltagesatbothofthecurrentoutputpinsbeasneargroundpotential(0VDC)aspossible.WithVREF=+10Veverymillivoltappearingatei-therIOUT1orIOUT2willcausea0.01%linearityerror.InmostapplicationsthisoutputcurrentisconvertedtoavoltagebyusinganopampasshowninFigure7.
Theinvertinginputoftheopampisa“virtualground”createdbythefeedbackfromitsoutputthroughtheinternal15k?re-sistor,Rfb.Alloftheoutputcurrent(determinedbythedigitalinputandthereferencevoltage)willflowthroughRfbtotheoutputoftheamplifier.Two-quadrantoperationcanbeob-tainedbyreversingthepolarityofVREFthuscausingIOUT1toflowintotheDACandbesourcedfromtheoutputoftheam-plifier.Theoutputvoltage,ineithercase,isalwaysequaltoIOUT1xRfbandistheoppositepolarityofthereferencevolt-age.
ThereferencecanbeeitherastableDCvoltagesourceoranACsignalanywhereintherangefrom?10Vto+10V.TheDACcanbethoughtofasadigitallycontrolledattenuator:theoutputvoltageisalwayslessthanorequaltotheappliedreferencevoltage.TheVREFterminalofthedevicepresentsanominalimpedanceof15k?togroundtoexternalcircuitry.AlwaysusetheinternalRfbresistortocreateanoutputvolt-agesincethisresistormatches(andtrackswithtempera-ture)thevalueoftheresistorsusedtogeneratetheoutputcurrent(IOUT1).
wherethedigitalinputisthedecimal(base10)equivalentoftheapplied8-bitbinaryword(0to255),VREFisthevoltageatpin8and15k?isthenominalvalueoftheinternalresis-tance,R,oftheR-2Rladdernetwork(discussedinSection2.1).
SeveralfactorsexternaltotheDACitselfmustbeconsid-eredtomaintainanalogaccuracyandarecoveredinsubse-quentsections.
2.1TheCurrentSwitchingR-2RLadder
Theanalogcircuitry,Figure6,consistsofasilicon-chromium(SiCrorSi-chrome)thinfilmR-2Rladderwhichisdepositedonthesurfaceoxideofthemonolithicchip.Asaresult,therearenoparasiticdiodeproblemswiththeladder(astheremaybewithdiffusedresistors)sothereferencevoltage,VREF,canrange?10Vto+10VevenifVCCforthedeviceis5VDC.
ThedigitalinputcodetotheDACsimplycontrolsthepositionoftheSPDTcurrentswitchesandsteerstheavailableladdercurrenttoeitherIOUT1orIOUT2asdeterminedbythelogicin-11
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FIGURE6.
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FIGURE7.
2.3OpAmpConsiderations
TheopampusedinFigure7shouldhaveoffsetvoltagenull-ingcapability(SeeSection2.5).
Theselectedopampshouldhaveaslowavalueofinputbiascurrentaspossible.Theproductofthebiascurrenttimesthefeedbackresistancecreatesanoutputvoltageer-rorwhichcanbesignificantinlowreferencevoltageapplica-tions.BI-FET?opampsarehighlyrecommendedforusewiththeseDACsbecauseoftheirverylowinputcurrent.Transientresponseandsettlingtimeoftheopampareim-portantinfastdatathroughputapplications.Thelargeststa-bilityproblemisthefeedbackpolecreatedbythefeedbackresistance,Rfb,andtheoutputcapacitanceoftheDAC.Thisappearsfromtheopampoutputtothe(?)inputandincludesthestraycapacitanceatthisnode.Additionofaleadcapaci-tance,CCinFigure8,greatlyreducesovershootandringingattheoutputforastepchangeinDACoutputcurrent.Finally,theoutputvoltageswingoftheamplifiermustbegreaterthanVREFtoallowreachingthefullscaleoutputvolt-age.Dependingontheloadingontheoutputoftheamplifierandtheavailableopampsupplyvoltages(only±12voltsinmanydevelopmentsystems),areferencevoltagelessthan10voltsmaybenecessarytoobtainthefullanalogoutputvoltagerange.
2.4BipolarOutputVoltagewithaFixedReference
Theadditionofasecondopamptothepreviouscircuitrycanbeusedtogenerateabipolaroutputvoltagefromafixedref-erencevoltage.This,ineffect,givessignsignificancetotheMSBofthedigitalinputwordandallowstwo-quadrantmulti-plicationofthereferencevoltage.Thepolarityoftherefer-encecanalsobereversedtorealizefull4-quadrantmultipli-cation:±VREFx±DigitalCode=±VOUT.ThiscircuitisshowninFigure9.
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Thisconfigurationfeaturesseveralimprovementsoverexist-ingcircuitsforbipolaroutputswithothermultiplyingDACs.Onlytheoffsetvoltageofamplifier1hastobenulledtopre-servelinearityoftheDAC.Theoffsetvoltageerrorofthesecondopamp(althoughaconstantoutputvoltageerror)hasnoeffectonlinearity.Itshouldbenulledonlyifabsoluteoutputaccuracyisrequired.Finally,thevaluesoftheresis-torsaroundthesecondamplifierdonothavetomatchthein-ternalDACresistors,theyneedonlytomatchandtempera-turetrackeachother.Athinfilm4-resistornetworkavailablefromBeckmanInstruments,Inc.(partno.694-3-R10K-D)isideallysuitedforthisapplication.Theseresistorsarematchedto0.1%andexhibitonly5ppm/?Cresistancetrack-ingtemperaturecoefficient.Twoofthefouravailable10k?resistorscanbeparalleledtoformRinFigure9andtheothertwocanbeusedindependentlyastheresistancesla-beled2R.
2.5ZeroAdjustment
Foraccurateconversions,theinputoffsetvoltageoftheout-putamplifiermustalwaysbenulled.AmplifieroffseterrorscreateanoveralldegradationofDAClinearity.
ThefundamentalpurposeofzeroingistomakethevoltageappearingattheDACoutputsasnear0VDCaspossible.ThisisaccomplishedforthetypicalDAC—opampconnec-tion(Figure7)byshortingoutRfb,theamplifierfeedbackre-sistor,andadjustingtheVOSnullingpotentiometeroftheopampuntiltheoutputreadszerovolts.Thisisdone,ofcourse,withanapplieddigitalcodeofallzerosifIOUT1isdrivingtheopamp(allone’sforIOUT2).TheshortaroundRfbisthenre-movedandtheconverteriszeroadjusted.
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tsOPAmpLF356LF351LF357*
CC22pF22pF10pF
(OtoFullScale)
4μs5μs2μs
*2.4k?RESISTORADDEDFROM?INPUTTOGROUNDTOINSURESTABILITY
FIGURE8.
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InputCode
MSB111000
110100
100110
100110
100110
100110
LSB100110
100110
FIGURE9.
2.6Full-ScaleAdjustment
InthecasewherethematchingofRfbtotheRvalueoftheR-2Rladder(typically±0.2%)isinsufficientforfull-scaleac-curacyinaparticularapplication,theVREFvoltagecanbeadjustedoranexternalresistorandpotentiometercanbeaddedasshowninFigure10toprovideafull-scaleadjust-ment.
Thetemperaturecoefficientsoftheresistorsusedforthisad-justmentareofanimportantconcern.Topreventdegrada-tionofthegainerrortemperaturecoefficientbytheexternal
+VREFIDEALVOUT?VREF*THESERESISTORSAREAVAILABLEFROMBECKMANINSTRUMENTS,INC.ASTHEIRPARTNO.694-3-R10K-D
resistors,theirtemperaturecoefficientsideallywouldhavetomatchthatoftheinternalDACresistors,whichisahighlyim-practicalconstraint.ForthevaluesshowninFigure10,iftheresistorandthepotentiometereachhadatemperaturecoef-ficientof±100ppm/?Cmaximum,theoverallgainerrortem-peraturecoefficentwouldbedegradedamaximumof0.0025%/?Cforanadjustmentpotsettingoflessthan3%ofRfb.
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2.7UsingtheDAC0830inaVoltageSwitchingConfiguration
TheR-2Rladdercanalsobeoperatedasavoltageswitch-ingnetwork.Inthismodetheladderisusedinaninvertedmannerfromthestandardcurrentswitchingconfiguration.
Thereferencevoltageisconnectedtooneofthecurrentout-putterminals(IOUT1fortruebinarydigitalcontrol,IOUT2isforcomplementarybinary)andtheoutputvoltageistakenfromthenormalVREFpin.Theconverteroutputisnowavoltageintherangefrom0Vto255/256VREFasafunctionoftheap-plieddigitalcodeasshowninFigure11.
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FIGURE10.AddingFull-ScaleAdjustment
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FIGURE11.VoltageModeSwitching
Thisconfigurationoffersseveralusefulapplicationadvan-tages.Sincetheoutputisavoltage,anexternalopampisnotnecessarilyrequiredbuttheoutputimpedanceoftheDACisfairlyhigh(equaltothespecifiedreferenceinputre-sistanceof10k?to20k?)soanopampmaybeusedforbufferingpurposes.SomeoftheadvantagesofthismodeareillustratedinFigures12,13,14,15.
TherearetwoimportantthingstokeepinmindwhenusingthisDACinthevoltageswitchingmode.Theappliedrefer-encevoltagemustbepositivesincethereareinternalpara-siticdiodesfromgroundtotheIOUT1andIOUT2terminalswhichwouldturnoniftheappliedreferencewentnegative.ThereisalsoadependenceofconversionlinearityandgainerroronthevoltagedifferencebetweenVCCandthevoltageappliedtothenormalcurrentoutputterminals.Thisisare-sultofthevoltagedriverequirementsoftheladderswitches.Toensurethatall8switchesturnonsufficiently(soasnottoaddsignificantresistancetoanylegoftheladderandtherebyintroduceadditionallinearityandgainerrors)itisrecommendedthattheappliedreferencevoltagebekeptlessthan+5VDCandVCCbeatleast9VmorepositivethanVREF.Theserestrictionsensurelessthan0.1%linearityandgainerrorchange.Figures16,17,18characterizetheef-fectsofbringingVREFandVCCclosertogetheraswellastypicaltemperatureperformanceofthisvoltageswitchingconfiguration.
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??
Voltageswitchingmodeeliminatesoutputsignalinver-sionandthereforeaneedforanegativepowersupply.Zerocodeoutputvoltageislimitedbythelowleveloutputsaturationvoltageoftheopamp.The2k?pull-downre-sistorhelpstoreducethisvoltage.
VOSoftheopamphasnoeffectonDAClinearity.
FIGURE12.SingleSupplyDAC
?
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DAC0830SeriesApplicationHints
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FIGURE13.ObtainingaBipolarOutputfromaFixed
ReferencewithaSingleOpAmp
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FIGURE14.BipolarOutputwithIncreasedOutputVoltageSwing
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