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5ADCEvaluation
5.1HardwareSelection
5.1.1AnalogInputSignalGenerator
5.1.2ClockSignalGenerator
ADCEvaluation
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ThissectiondescribeshowtosetupatypicalADCevaluationsystemthatissimilartowhatTIusesto
performtestingfordata-sheetgeneration.Consequently,theinformationinthissectionisgenericinnature
andisapplicabletoallhigh-speed,high-resolutionADCevaluations.Thissectioncoverssignaltone
analysis,whichyieldsADCdata-sheetfiguresofmeritsuchassignal-to-noiseratio(SNR)andspurious
freedynamicrange(SFDR).
TorevealthetrueperformanceoftheADCunderevaluation,greatcareshouldbetakeninselectingboth
theADCsignalsourceandADCclockingsource.
WhenchoosingthequalityoftheADCanaloginputsource,considerbothharmonicdistortionperformance
ofthesignalgeneratorandthenoiseperformanceofthesource.
Inmanycases,theharmonicdistortionperformanceofthesignalgeneratorisinferiortothatoftheADC,
andadditionalfilteringisneededifusersexpecttoreproducetheADCSFDRnumbersfoundinthedata
sheet.Userscaneasilyevaluatetheharmonicdistortionofthesignalgeneratorbyhookingitdirectlytoa
spectrumanalyzer,measuringthepoweroftheoutputsignal,andcomparingthattothepowerofthe
integermultiplesoftheoutputsignalfrequency.IftheharmonicdistortionisworsethantheADCunder
evaluation,theADCdigitizestheperformanceofthesignalgeneratorandthetrueSFDRoftheADCis
masked.Toalleviatethis,itisrecommendedthatusersprovideadditionalLCfilteringafterthesignal
generatoroutput.
Anotherimportantmetricwhendecidingonasignalgeneratorisitsnoiseperformance.Aswiththe
distortionperformance,ifthenoiseperformanceisworsethanthatoftheADCunderevaluation,theADC
digitizestheperformanceofthesource.Noisecanbebrokenintotwocomponents,broadbandnoiseand
close-inphasenoise.BroadbandnoisecanbeimprovedbytheLCfilteraddedtoimprovedistortion
performance;however,theclose-inphasenoisetypicallycannotbeimprovedbyadditionalfiltering.
Therefore,whenselectingananalogsignalsource,itisimportanttoreviewthemanufacturer'sphase
noiseplotsandtakecaretochooseasignalgeneratorwiththebestphase-noiseperformance.
Equallyimportantinthehigh-performanceADCevaluationsetupistheselectionoftheclockingsource.
MostmodernADCs,theADS61xx/ADS61B23included,accepteitherasinusoidalorasquare-waveclock
input.Thekeymetricinselectingaclockingsourceisselectingasourcewiththelowestjitter.This
becomesincreasinglyimportantastheADCinputfrequency(f
in
)increases,becausetheADCSNR
evaluationsetupscanbecomejitter-limited(t
j
)asshownbythefollowingequation.
SNR(dBc)=20log(2π×f
in
×t
j
(rms))
Intheory,asquare-wavesourcewithfemtosecondjitterwouldbeidealforanADCevaluationsetup.
However,inpracticalterms,mostcommerciallyavailablesquare-wavegeneratorsofferjittermeasuredin
picoseconds,whichistoogreatforhigh-resolutionADCevaluationsetups.Therefore,mostevaluation
setupsrelyontheADCinternalclockbuffertoconvertasinusoidalinputsignalintoaultralow-jittersquare
wave.Whenselectingasinusoidalclockingsource,ithasbeenshownthatphasenoisehasadirect
impactonjitterperformance.Consequently,greatscrutinyshouldbeappliedtothephase-noise
performanceoftheclockingsignalgenerator.TIhasfoundthathigh-Qmonolithiccrystalfilterscan
improvethephasenoiseofthesignalgenerator,andthesefiltersbecomeessentialelementsofthe
evaluationsetupwhenhighADCinputfrequenciesarebeingevaluated.
14SLAU206B–September2007–RevisedApril2008
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