MAX038A说明书.docx
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MAX038A说明书.docx
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MAX038A说明书
VersatileWaveformGeneratorOperatesfrom0.1Hzto20MHz
Asasingle-chipsourceofhigh-frequencywaveforms,theMAX038canservebothasthecoreofamoderate-performancewaveformgenerator,andasacard-levelwaveformsourceforelectronicsystemsinwhichthecostofadedicatedwaveformgeneratormaybeprohibitive.
TheMAX038isaprecision,high-frequencyfunctiongeneratorthatproducesaccuratesine,square,triangle,sawtooth,andpulsewaveformswithaminimumofexternalcomponents.Theinternal2.5Vreference(plusanexternalcapacitorandpotentiometer)letsyouvarythesignalfrequencyfrom0.1Hzto20MHz.Anapplied±2.3Vcontrolsignalvariesthedutycyclebetween10%and90%,enablingthegenerationofsawtoothwaveformsandpulse-widthmodulation.
Asecondfrequency-controlinput-usedprimarilyasaVCOinputinphase-locked-loopapplications-provides±70%offinecontrol.Thiscapabilityalsoenablesthegenerationoffrequencysweepsandfrequencymodulation.Thefrequencyandduty-cyclecontrolshaveminimalinteractionwitheachother.
Alloutputamplitudesare2Vp-p,symmetricalaboutground.Thelow-impedanceoutputterminaldeliversasmuchas±20mA,andatwo-bitcodeappliedtotheTTL-compatibleA0andA1inputsselectsthesine,square,ortriangleoutputwaveform:
TosynchronizeMAX038operationwithotherdevicesinthesystem,theinternaloscillatorproducesaTTL-compatibleSYNCoutput,whosedutycycleremainsconstantat50%regardlessofthedutycyclesetfortheoutputwaveform.TheMAX038'sinternalphasedetectorenablessuchsynchronizationaswell.Italsoenablesthedemodulationoffrequency-modulatedsignals.
DetailsofOperation
Byalternatelycharginganddischarginganexternalcapacitor,theMAX038'srelaxationoscillatorproducessimultaneoussquareandtrianglewaves.Aninternalsine-shapingcircuitconvertsthetriangletoalow-distortion,constant-amplitudesinewave.Thesine,square,andtrianglewavesareappliedtoaninternalmultiplexerthatletsyouselecttheoutputwaveformaccordingtothestateofaddresslinesA0andA1.Theoutputamplituderemainsconstantat±1Vregardlessofwaveshapeorfrequency(Figure1).
Figure1.AmongwaveformsavailableattheMAX038outputarethesine(a,b),triangle(c,d),andsquare(e,f).
SeeFigure2fortheMAX038'sblockdiagramandoperatingcircuit.Poweredfrom±5V,thedeviceconsumes400mWandhasanominaloutputfrequencysetbytheoscillatorcapacitorCF.CoarsedeviationsfromthatfrequencyaremadebyvaryingtheIINcurrentfrom2µAto750µA,arangeof375:
1(Figure3).Asshown,theIINcurrentcanbederivedusingtheon-board2.5Vreferenceandanexternalfixedorvariableresistor.
Figure2.ThisfigurecombinestheblockdiagramandbasicoperatingcircuitfortheMAX0380.1Hz-to-20MHzwaveformgenerator.
Figure3.TheoutputfrequencyinFigure2varieswithcurrentintoIINandthevalueofCF.
Toadjustthefrequencydigitally,connectavoltage-outputDACtoIINviaaseriesresistor(Figure4).Theconverteroutputrangesfrom0Vatzeroto2.5(255/256)Vatfull-scale.CurrentinjectedbytheconverterintoIIN,therefore,rangesfrom0µAto748µA.The2.5Vreferenceand1.2MΩresistorinjectaconstant2µA,so(bysuperposition)thenetcurrentintoIINrangesfrom2µA(atacodeof00000000)to750µA(at11111111).Thequad-DACICoperatesfrom5Vor±5V.Asdescribedbelow,itcanalsoprovidedigitalcontrolofFADJandDADJ.
Figure4.Three8-bitD/Aconverterseasilyprovidedigitalcontrolofthetwofrequency-adjustinputs(IINandFADJ)andtheduty-cycle-adjustinput(DADJ).
Forfineadjustments(±70%),applyacontrolvoltageintherange±2.3Vtothefrequencyadjust(FADJ)terminal(Figure5).BothFADJandIINhavewidebandwidthsthatallowtheoutputfrequencytobemodulatedatamaximumrateofabout2MHz(Figure6).Asthemorelinearinput,IINispreferredforopen-loopfrequencycontrol.Asthevoltageinput,FADJisbettersuitedforuseinaphase-lockedloop.FordigitalcontrolofFADJ,configureaDACandexternalopamp(asinFigure4)toproduceanoutputrangingfrom-2.3V(00000000)to2.3V(11111111).
Figure5.Forfinecontroloftheoutputfrequency,applyacontrolvoltageintherange±2.3VtoFADJ.
Figure6.TheMAX038inputsIINandFADJallowgross(a)andfine(b)adjustmentsofoutputfrequency.
Dutycycle(thepercentageoftimethattheoutputispositive)canbeadjustedintherange10%to90%byapplyinga±2.3Vcontrolsignaltotheduty-cycle-adjustterminalDADJ(Figure7a).ThissignalchangestheratioofchargecurrenttodischargecurrentfortheCFcapacitorwhilemaintaininganearlyconstantoutputfrequency(Figure7b).
Figure7.AnindependentDADJcontrolvoltageintherange±2.3Vadjuststhedutycycle(a)withlittleeffectonfrequency(b).
TheDADJinputalsoletsyouminimizedistortionintheoutputsinewave.Minimumdistortionoccursatadutycycleofexactly50%,butthetypicaldutycycle(withVDADJ=0V)is50%±2%.Byapplyingasmallcontrolvoltage(typicallylessthan±100mV)toDADJ,therefore,youcansettheexact50%symmetrythatminimizesdistortion(seeinsert,Figure8).
Figure8.Thiscircuitletsyouadjustfortheminimumsine-wavedistortionavailableata50%dutycycle.
ThesourcedrivingDADJmustsupplyaconstant250µA(seeFigure2).Thetemperaturecoefficientofthisinternalcurrentsinkisunimportantforopampsandotherlow-impedancesources,butissignificantwhenusingavariableresistorasshown.Thus,variableresistorssuitmanualoperationonly,inwhichtheoperatorcancorrecterrorsthroughreadjustment.LikeFADJ,DADJhasa2MHzbandwidthand±2.3Vrange.Itcanbedigitallycontrolledwithanidenticalcircuit(Figure4).Figure9showstheduty-cyclemodulationthatresultswhenatrianglewaveisappliedtoDADJ.
Figure9.AtrianglewaveappliedtoDADJprovideslinearvariationsinthedutycycle.
Phase-lockedOperation
TheMAX038'sinternalphasedetectorisintendedprimarilyforuseinphase-locked-loop(PLL)configurations.InFigure10a,forexample,thephasedetectorinIC2enablesthatdevicetosynchronizeitsoperationwiththatofIC1.YouconnecttheappliedreferencesignaltoIC2'sTTL/CMOS-compatiblephase-detectorinput(PDI)andconnectthephase-detectoroutput(PDO)totheinput(FADJ)oftheinternalvoltage-controlledoscillator.PDOistheoutputofanexclusive-ORgate-amixer-whichproducesrectangularcurrentwaveformsatfrequenciesequaltothesumanddifferenceofthePDIfrequencyandtheMAX038outputfrequency.ThesewaveformsareintegratedbyCPDtoformatriangle-wavevoltageoutputatPDO(Figure10b).The10Ω/100pFpairatPDIlimitsthatpin'srateofriseto10ns.
Figure10.TheSYNCoutputletsyousynchronizeoneMAX038toanother.
ThePDOcurrent-pulselevelsare0µAand500µA,withadutycyclethatapproaches50%whenPDIandtheoutputareinphasequadrature(90°outofphase).Otherwise,thedutycycleapproaches100%whenthephasedifferenceapproaches180°,and0%whenthephasedifferenceapproaches0°.RPD,CPD,andRZcompriseafilterthatdeterminesthePLLfrequencyresponse.
AttheSYNCoutputisasquarewaveoffixed50%dutycycle,whoserisingedgecoincideswiththerisingedgeofanoutputsineortrianglewaveasitpassesthroughzerovolts.Iftheoutputisasquarewave,SYNC'srisingedgeoccursatthemid-pointofthepositiveportion,causingSYNCtoleadtheoutputby90°.
SYNCletsyouslaveoneMAX038toanotherbyprovidingaTTL-compatiblesquarewaveatthephase-detectorinput(PDI),asrequiredbytheslaveddevice(IC2).Ontheotherhand,SYNCisn'tavailableifaMAX038issynchronizedwithsineortrianglewavesfromothersources.Forthosecases,thePDIinputmustbedrivenbyacomparator(asshowninFigure11)tosquareupthesignalandprovidetheappropriatelevelshifting.
ClickforLargerImage
Figure11.Thiscircuitmodulatesa10MHzcarrierwith10kHz,andthenrecoversthemodulatingsignal.
Theinternalphasedetectorcanalsodemodulatefrequency-modulatedsignals(Figure11).Inthiscircuit,theoutputofoneMAX038isbeingfrequencymodulatedwitha10kHzsinewave.The±34mVsignalatFADJofIC1causesa±1%variationintheoutputfrequency(±100kHz),andtherateofvariationis10kHz.ThecomparatorassuresapropersquarewaveforIC3'sPDIinputasmentionedabove.ThetwoMAX038saresettothesamecenterfrequency.
ThefrequenciesatIC3'sphase-detectoroutputarethesumanddifferenceofthefrequenciesatPDIandOUT.Thus,withappropriatecutofffrequencyandgain,thelowpassfilter(IC4)passesonlytheoriginal10kHzsignaltothedemodulatedoutput.Thepoleforthisfilterissetbythe16.2kΩand100pFcomponents.AsinFigure10a,thefrequencyresponseforIC3'sPLLissetbyRPD,CPD,andRZ(seeSidebar).
Whentheloopisinlock,PDIisinapproximatephasequadraturewiththeoutputsignal.Alsowheninlock,thedutycycleatPDOis50%,andPDO'saverageoutputcurrentis250µA.ThecurrentsinkatFADJdemandsaconstant250µA,soPDOoutputsaboveandbelowthatleveldevelopabipolarerrorvoltageacrossRPDthatdrivestheFADJvoltageinput.Note-theMAX038'sinternalphasedetectorisaphase-onlydetector,producingaPLLwhosefrequency-capturerangeislimitedbythebandwidthofitsloopfilter.Forwider-rangeapplications,consideranexternalphase-frequencydetector.
Togaintheadvantagesofawidercapturerangeandanoptional÷Ncircuit(whichallowsthePLLtolockontoarbitrarymultiplesoftheappliedfrequency),youcanintroduceanexternalfrequency-phasedetectorsuchasthe74HC4046orthediscrete-gatev
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- MAX038A 说明书