requirementsdgl-028

MT-028TUTORIALVoltage-to-FrequencyConvertersbyWaltKesterandJamesBryantINTRODUCTIONAvoltage-to-frequencyconverter(VFC)isanoscillatorwhosefrequencyislinearlyproportionaltoacontrolvoltage.
TheVFC/counterADCismonotonicandfreeofmissingcodes,integratesnoise,andcanconsumeverylittlepower.
Itisalsoveryusefulfortelemetryapplications,sincetheVFC,whichissmall,cheapandlow-poweredcanbemountedontheexperimentalsubject(patient,wildanimal,artilleryshell,etc.
)andcommunicatewiththecounterbyatelemetrylinkasshowninFigure1.
CONNECTIONNEEDNOTBEDIRECTCIRCUITISIDEALFORTELEMETRYANALOGINPUTOUTPUTVFCFREQUENCYCOUNTERFigure1:Voltage-to-FrequencyConverter(VFC)andFrequencyCounterMakeaLow-Cost,Versatile,High-ResolutionADCTherearetwocommonVFCarchitectures:thecurrent-steeringmultivibratorVFCandthecharge-balanceVFC(Reference1).
Thecharge-balancedVFCmaybemadeinasynchronousorsynchronous(clocked)forms.
TherearemanymoreVFO(variablefrequencyoscillator)architectures,includingtheubiquitous555timer,butthekeyfeatureofVFCsislinearity—fewVFOsareverylinear.
Thecurrent-steeringmultivibratorVFCisactuallyacurrent-to-frequencyconverterratherthanaVFC,but,asshowninFigure2,practicalcircuitsinvariablycontainavoltage-to-currentconverterattheinput.
Theprincipleofoperationisevident:thecurrentdischargesthecapacitoruntilathresholdisreached,andwhenthecapacitorterminalsarereversed,thehalf-cyclerepeatsitself.
Thewaveformacrossthecapacitorisalineartriangularwave,butthewaveformoneitherterminalwithrespecttogroundisthemorecomplexwaveformshown.
Rev.
A,10/08,WKPage1of7MT-028Figure2:ACurrent-SteeringVFCPracticalVFCsofthistypehavelinearitiesaround14-bits,andcomparablestability,althoughtheymaybeusedinADCswithhigherresolutionswithoutmissingcodes.
Theperformancelimitsaresetbycomparatorthresholdnoise,thresholdtemperaturecoefficient,andthestabilityanddielectricabsorption(DA)ofthecapacitor,whichisgenerallyadiscretecomponent.
Thecomparator/voltagereferencestructureshowninthediagramismoreofarepresentationofthefunctionperformedthantheactualcircuitused,whichismuchmoreintegratedwiththeswitching,andcorrespondinglyhardertoanalyze.
ThistypeofVFCissimple,inexpensive,andlow-powered,andmostrunfromawiderangeofsupplyvoltages.
TheyareideallysuitedforlowcostmediumaccuracyADCanddatatelemetryapplications.
ThechargebalanceVFCshowninFigure3ismorecomplex,moredemandinginitssupplyvoltageandcurrentrequirements,andmoreaccurate.
Itiscapableof16-18bitlinearity.
Page2of7MT-028+–PRECISIONONE-SHOTIFREQUENCYCOUNTERVREFPRECISIONCURRENTSOURCEANALOGINPUTFREQUENCYOUTPUTDIGITALOUTPUTFigure3:ChargeBalanceVoltage-to-FrequencyConverter(VFC)TheintegratorcapacitorischargedbythesignalasshowninFigure3.
Whenitpassesthecomparatorthreshold,afixedchargeisremovedfromthecapacitor,buttheinputcurrentcontinuestoflowduringthedischarge,sonoinputchargeislost.
Thefixedchargeisdefinedbytheprecisioncurrentsourceandthepulsewidthoftheprecisionmonostable.
Theoutputpulserateisthusaccuratelyproportionaltotherateatwhichtheintegratorchargesfromtheinput.
Atlowfrequencies,thelimitsontheperformanceofthisVFCaresetbythestabilityofthecurrentsourceandthemonostabletiming(whichdependsonthemonostablecapacitor,amongotherthings).
Theabsolutevalueandtemperaturestabilityoftheintegrationcapacitordonotaffecttheaccuracy,althoughitsleakageanddielectricabsorption(DA)do.
Athighfrequencies,second-ordereffects,suchasswitchingtransientsintheintegratorandtheprecisionofthemonostablewhenitisretriggeredverysoonaftertheendofapulse,taketheirtollonaccuracyandlinearity.
Thechangeoverswitchinthecurrentsourceaddressestheintegratortransientproblem.
Byusingachangeoverswitchinsteadoftheon/offswitchmorecommononolderVFCdesigns:(a)therearenoon/offtransientsintheprecisioncurrentsourceand(b)theoutputstageoftheintegratorseesaconstantload—mostofthetimethecurrentfromthesourceflowsdirectlyintheoutputstage;duringchargebalance,itstillflowsintheoutputstage,butthroughtheintegrationcapacitor.
Thestabilityandtransientbehavioroftheprecisionmonostablepresentmoreproblems,buttheissuemaybeavoidedbyreplacingthemonostablewithaclockedbistablemultivibrator.
ThisarrangementisknownasasynchronousVFCorSVFCandisshowninFigure4.
Page3of7MT-028+–IFREQUENCYCOUNTERVREFPRECISIONCURRENTSOURCEANALOGINPUTFREQUENCYOUTPUTDIGITALOUTPUTDFLIP-FLOPDCLOCKINPUTFigure4:SynchronousVFC(SVFC)Thedifferencefromthepreviouscircuitisquitesmall,butthechargebalancepulselengthisnowdefinedbytwosuccessiveedgesoftheexternalclock.
Ifthisclockhaslowjitter,thechargewillbeveryaccuratelydefined.
Theoutputpulsewillalsobesynchronouswiththeclock.
SVFCsofthistypearecapableofupto18-bitlinearityandexcellenttemperaturestability.
Thissynchronousbehaviorisconvenientinmanyapplications,sincesynchronousdatatransferisofteneasiertohandlethanasynchronous.
Itdoesmean,however,thattheoutputofanSVFCisnotapuretone(plusharmonics,ofcourse)likeaconventionalVFC,butcontainscomponentsharmonicallyrelatedtotheclockfrequency.
ThedisplayofanSVFCoutputonanoscilloscopeisespeciallymisleadingandisacommoncauseofconfusion—achangeofinputtoaVFCproducesasmoothchangeintheoutputfrequency,butachangetoanSVFCproducesachangeinprobabilitydensityofoutputpulsesNandN+1clockcyclesafterthepreviousoutputpulse,whichisoftenmisinterpretedasseverejitterandasignofafaultydevice(seeFigure5).
Page4of7MT-028VFCSVFCSVFCCLOCKTVARIESSMOOTHLYWITHINPUTTT1T2THERELATIVEPROBABILITYOFDELAYT1ORT2VARIESWITHINPUT,BUTBOTHDELAYSDOOCCURFigure5:VFCandSVFCWaveformsAnotherproblemwithSVFCsisnonlinearityatoutputfrequenciesrelatedtotheclockfrequency.
IfwestudythetransfercharacteristicofanSVFC,wefindnonlinearitiesclosetosub-harmonicsoftheclockfrequencyFCasshowninFigure6.
TheycanbefoundatFC/3,FC/4,andFC/6.
Thisisduetostraycapacitanceonthechip(andinthecircuitlayout!
)andthecouplingoftheclocksignalintotheSVFCcomparatorwhichcausesthedevicetobehaveasaninjection-lockedphase-lockedloop(PLL).
ThisproblemisintrinsictoSVFCs,butisnotoftenserious:ifthecircuitcardiswelllaidout,andclockamplitudeandrateofchangekeptaslowaspractical,theeffectisadiscontinuityinthetransfercharacteristicoflessthan8LSBs(at18-bitresolution)atFC/3andFC/4,andlessatothersub-harmonics.
Thisisfrequentlytolerable,sincethefrequencieswhereitoccursareknown.
Ofcourse,ifthecircuitlayoutordecouplingispoor,theeffectmaybemuchlarger,butthisisthefaultofpoordesignandnottheSVFCitself.
Page5of7MT-028NonlinearitiescausedbyinjectionlockingduetoclockfeedthroughShouldonlybe6-8LSBsat18-bitswithproperlayoutanddecouplingNonlinearitiesoccuratsubharmonicsoftheclock,whereFS=Fclock/2Figure6:SVFCNonlinearityItisevidentthattheSVFCisquantized,whilethebasicVFCisnot.
ItdoesNOTfollowfromthisthatthecounter/VFCADChashigherresolution(neglectingnonlinearities)thanthecounter/SVFCADC,becausetheclockinthecounteralsosetsalimittotheresolution.
WhenaVFChasalargeinput,itrunsquicklyand(countingforashorttime)givesgoodresolution,butitishardtogetgoodresolutioninareasonablesampletimewithaslow-runningVFC.
Insuchacase,itmaybemorepracticaltomeasuretheperiodoftheVFCoutput(thisdoesnotworkforanSVFC),butofcoursetheresolutionofthissystemdeterioratesastheinput(andthefrequency)increases.
However,ifthecounter/timerarrangementismade"smart,"itispossibletomeasuretheapproximateVFCfrequencyandtheexactperiodofnotone,butNcycles(wherethevalueofNisdeterminedbytheapproximatefrequency),andmaintainhighresolutionoverawiderangeofinputs.
TheAD1170modularADCreleasedin1986isanexampleofthisarchitecture.
VFCshavemoreapplicationsthanasacomponentinADCs.
Sincetheiroutputisapulsestream,itmayeasilybesentoverawiderangeoftransmissionmedia(PSN,radio,optical,IR,ultrasonic,etc.
).
Itneednotbereceivedbyacounter,butbyanotherVFCconfiguredasafrequency-to-voltageconverter(FVC).
Thisgivesananalogoutput,andaVFC-FVCcombinationisaveryusefulwayofsendingaprecisionanalogsignalacrossanisolationbarrier.
ThereareanumberofissuestobeconsideredinbuildingFVCsfromVFCs,andtheseareconsideredinReferences2-5.
Page6of7Page7of7MT-028SUMMARYAnalogDeviceshasawiderangeofvoltagetofrequencyconverters(VFCs)fortheinstrumentation,industrialandautomationmarkets(seeVoltage-to-FrequencyConverterSelectionTable),includingtheAD537,AD650,AD652,AD654,andADVFC32.
Theyareideallysuitedforuseinanalog-to-digitalconversion(ADC),longtermintegration,linearfrequencymodulationanddemodulation,andfrequency-to-voltageconversion.
AnalogDevices'latestfamilyofVFCs,theAD7740,AD7741,andAD7742,aresynchronousVFCsbasedonsigma-deltatechnologyandprovidehighlinearityintinypackagesatlowcost.
REFERENCES1.
JohnL.
Lindesmith,"Voltage-to-DigitalMeasuringCircuit,"U.
S.
Patent2,835,868,filedSeptember16,1952,issuedMay20,1958.
(voltage-to-frequencyADC).
2.
PaulKlonowski,"Analog-to-DigitalConversionUsingVoltage-to-FrequencyConverters,"ApplicationNoteAN-276,AnalogDevices,Inc.
(agoodapplicationnoteonVFCs).
3.
JamesM.
Bryant,"Voltage-to-FrequencyConverters,"ApplicationNoteAN-361,AnalogDevices,Inc.
(agoodoverviewofVFCs).
4.
WaltJung,"OperationandApplicationsoftheAD654ICV-FConverter,"ApplicationNoteAN-278,AnalogDevices,Inc.
5.
SteveMartin,"UsingtheAD650Voltage-to-FrequencyConverterasaFrequency-to-VoltageConverter,"ApplicationNoteAN-279,AnalogDevices,Inc.
(adescriptionofafrequency-to-voltageconverterusingthepopularAD650VFC).
6.
WaltKester,Analog-DigitalConversion,AnalogDevices,2004,ISBN0-916550-27-3,Chapter3.
AlsoavailableasTheDataConversionHandbook,Elsevier/Newnes,2005,ISBN0-7506-7841-0,Chapter3.
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