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2CircuitDescription
2.1SchematicDiagram
2.2ADCCircuitFunction
2.2.1ADCOperationalMode
2.2.2EVMPowerConnections
2.2.3ADCAnalogInputs
CircuitDescription
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TheschematicdiagramfortheEVMisinSection6.3.
Thefollowingsectionsdescribethefunctionofindividualcircuits.Seetherelevantdatasheetfordevice
operatingcharacteristics.
Bydefault,theADCisconfiguredtooperateinparallel-modeoperation,becausejumper(J3)assertsa
3.3-VstatetotheADCresetpin.Consequently,theSW1resetpushbuttonmustbepressedonlywhenthe
deviceisconfiguredinserialoperationmode.BecausetheADCisinparalleloperationmode,voltages
areusedtosettheADCconfigurationmodes.UserscanusetheEVMsilkscreentosettheoperation
modes.
PowerissuppliedtotheEVMbybananajacksockets.Separateconnectionsareprovidedfora3.3-V
digitalbuffersupply(J11)and3.3-Vanalogsupply(J13);however,bydefaulttheseareshortedtogether
usingR65,a0-Ωresistor.Consequently,userscansupplypowertoeitherJ11orJ13topowertheADC.
TheseparateconnectionsallowuserstoseparateanaloganddigitalsuppliesbyremovingR65.When
usingtheamplifierevaluationpath,connectthepositiverailtoJ20andthenegativerailtoJ16.The
voltagesdependonthecouplingmethodandconnectiontotheADC.IftheADCVCMisnotsuppliedto
theamplifierandtheamplifierisconnectedtotheADCinadc-coupledfashion,setJ20to4VandJ16to
–1V.Inac-coupledconfigurationswheretheADCVCMbiasestheADCinputs,connectJ20to5Vand
J16toGND.TheADCSPIinterfaceandCDCP1803alsoarepoweredthroughJ20,whichshouldbeset
to5Vforoperationofthosecircuits.
TheEVMisconfiguredtoacceptasingle-endedinputsourceandconvertittoanac-coupleddifferential
signalusingatransformer.TheinputstotheADCmustbedc-biased,whichisaccomplishedbyusingthe
ADCVCMoutput.TheinputisprovidedbytheSMAconnectorJ8.
UsingSMAinputJ10,userscanevaluatetheADCusingaTHS4509amplifier,whichconvertsa
single-endedinputintoadifferentialsignalwhileproviding10dBofsignalgain.Usersshouldenablethe
amplifierpathbyconnectingJP71–2andbyshortingpositions2–3onbothsurface-mountjumpersJP5
andJP6.Atlowinputfrequencies,theADCrepresentsahigh-inputimpedanceandR38,R46,andC76
formalow-passfilterwitha3-dbcutofffrequencyof70MHz.Userscanchangethesecomponentvalues
dependingonthebandwidthofthesignaltheyaredigitizingtoband-limittheinputnoiseintotheADC.
UsinganexcessivelyhighcutofffrequencydegradestheSNRofthesystem.Beforebeginningevaluation
oftheamplifierpath,ausermustchoosewhethertodc-coupleorac-coupletheamplifierpath.
Inadc-coupledsystem,replaceC75andC77with0-ΩresistorsandremoveR37andR45.UsetheADC
VCMtosettheCMinputoftheamplifierbyensuringthatR21ispopulatedwitha0-Ωresistor.Because
theADChasacommon-modevoltageof1.5VandbecausetheTHS4509isnotarail-to-railamplifier,
adjustVCCto4Vand–VCCto–1V,whichcanbedonebyapplyingtherespectivevoltagestoJ20and
J16.
Foranac-coupledsystem,usethevoltagedividerR37andR45tosetthecommon-modeinputofthe
amplifier,whichshouldbesettothemidpointoftheamplifiersupply.Alternatively,userscanleaveR37
andR45unpopulatedandtheamplifiersetsitsowncommonvoltageto(VCC–VEE)/2.CapacitorsC75
andC77provideac-couplingofthesystem,andtheADCinputsthencanbebiasedbytheR41andR42
combination.Anotherac-coupledapproach,notsupportedonthisEVM,istouseatransformeratthe
outputsoftheTHS4509.Inthiscase,thetransformerprovidesforac-coupling,andtheinputsoftheADC
canbebiasedbyfeedingtheADCVCMtothetransformercentertaponthesecondary.
6SLAU206B–September2007–RevisedApril2008
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