AN-1447 Improving PSRR and CMRR in Fully …

1 ApplicationReportSNOA467A April2006 (PSRR)andCommonModeRejectionRatio(CMRR) ,a ,wewilllookatsomeoftheeffectsthatexterna lcomponentshaveonPSRRandCMRR:mismatchede xternalgain-settingresistors,theroleofth ebypasscapacitoronPSRR, (PSRR).. (CMRR).. April2006 RevisedJuly2006AN-1447 ImprovingPSRRandCMRRin FullyDifferentialAmplifiersSubmitDocumen tationFeedbackCopyright 2006,TexasInstrumentsIncorporatedRL8:-+V O2VO1+--IN+INBiasSHUTDOWNBYPASSVDDCS1 PF+Rf2Rf1Ri1Ri2+Input+ (PSRR)andCommonModeRejectionRatio(CMRR)a rethetwokeyspecificationswhenit ,a ,wewilllookatsomeoftheeffectsthatexterna lcomponentshaveonPSRRandCMRR:mismatchede xternalgain-settingresistors,theroleofth ebypasscapacitoronPSRR, ,a typicalfullydifferentialamplifierblockdi agramis outofphasewithrespecttoeachother,creatin gaBridge-Tied-Load(BTL) , FullyDifferentialAmplifiersSNOA467A April2006 RevisedJuly2006 SubmitDocumentationFeedbackCopyright 2006,TexasInstrumentsIncorporatedRL8:-+V O2VO1+--IN+INBiasSHUTDOWNBYPASSVDD + Vripple (ac)CS1 PF+Rf2Rf1Ri1Ri2+CBSpectrum Analyzeror audio Precision10.

2 Ci1Ci220 LogPSRR = Vout (ac) (PSRR)ThetypeofPSRR weareinterestedin hereis anACspecificationmeasuringtheabilityofth eamplifiertorejectAC-ripplevoltageonthep owersupplybus,asopposedtoa DCspecificationwherewemeasurethechangein outputvoltagefora changein ,ripplePSRRis theratioofthedifferentialoutputvoltageto thesupplyripplevoltageexpressedin dBasshownin Equation1.(1) testcircuitformeasuringPSRRis shownin 'sACinputsareterminatedtoGNDwitha lowvalueresistor typically10 . ,in therangeof100mVppto500mVppis combinedwithVDD. ThisACplusDCsignalis appliedtotheamplifier' a functionoftheamplifier' measuredwithanFFT(usinga spectrumanalyzerorAudioPrecisioninstrume nt).

3 UsingEquation1 April2006 RevisedJuly2006AN-1447 ImprovingPSRRandCMRRin FullyDifferentialAmplifiersSubmitDocumen tationFeedbackCopyright 2006,TexasInstrumentsIncorporatedRL8:-+V O2VO1+--IN+INBiasSHUTDOWNBYPASSVDDCS1 PF+Rf2Rf1Ri1Ri2+CBAC Volt Meteror audio PrecisionVcmiCMRR = 20 LogVout (ac)Vcmi (ac) (CMRR)CommonModeRejectionRatiois anabilityofthedifferentialamplifiertorej ectcommonmodeinputsignalandis expressedin dBasshownin Equation2.(2)A testcircuitformeasuringCMRRis shownis ' givestheCMRR measurementin ,CMRR measurementis sidenote,a lotofneweraudiomeasuringinstrumentssucha sAudioPrecisioncanperformtheCMRR test(anda lotofotheraudioparametersaswell) is , FullyDifferentialAmplifiersSNOA467A April2006 RevisedJuly2006 SubmitDocumentationFeedbackCopyright 2006,TexasInstrumentsIncorporated0405065 80 CMRR (dB)GAIN RESISTORS-MISMATCH |Rf1-Rf2| (:) 5V, RL=8:Vo2 - Vo1 =Ri2 + Rf2Ri1 + Rf1 VDD2 Ri1Ri2 - :a differentialamplifieranda common- 'hastwoequal halves.

4 Equation3 canbeobtainedbythesimpleanalysisofthedif ferentialamplifierin Figure1 withVcmi= (RiandRf) (Rf1andRf2)orinputresistors(Ri1andRi2).( 3)Ideally,whenRi1= Ri2andRf1= Rf2, , forexample,weassume:Ri1= Ri2= RRf1= ,with20%tolerancein s quitea bigDCoffsetvoltage!Asa result, a shownin thegraph,a alsoaffectedbythegainresistorsmismatch,b utwitha smallcapacitoronthebypasspinthiseffectis ,forallpracticalpurpose,resistorsof1% April2006 RevisedJuly2006AN-1447 ImprovingPSRRandCMRRin FullyDifferentialAmplifiersSubmitDocumen tationFeedbackCopyright 2006,TexasInstrumentsIncorporated20-100- 90-80-70-60-50-40-30-20-100 PSRR (dB)1k20k100kFREQUENCY (Hz)100CB= NoneCB= 1uFLM4928 VDD= 5V, RL=8 optional bypasscapacitor(CB)

5 Andallowa , ,a commonmodeamplifieradjuststhetwooutputst oa ,a bypasscapacitoris therealworld, ,mismatchin theexternalresistorscanresultin Figure5 atlowripplefrequencies,thePSRRis closeto70dB,evenwithouta ,usinga bypasscapacitorin uFoffersa , ,in cell-phonemarketwhereanexcellentPSRRis desired,particularlyatlowerfrequencies,a ddinga , ,it usedwhencombinedwiththefollowinggenerals uggestions:a)Makeuseofsymmetricalplaceme ntofcomponents( ).b)If notusinggroundplanes, )If a heat-sinkplaneis desiredforexposed-DAPpackages,it ) )If possible,maketheoutputtracesshortandequa lin FullyDifferentialAmplifiersSNOA467A April2006 RevisedJuly2006 SubmitDocumentationFeedbackCopyright 2006, ) If usinga starground,alltheGNDssuchasbypasscapacit orGND,supplycapacitorGNDs,andanyothersys temgroundshouldbedirectlyconnectedtoa singlepointontheboard,possiblyattheboard ' , April2006 RevisedJuly2006AN-1447 ImprovingPSRRandCMRRin FullyDifferentialAmplifiersSubmitDocumen tationFeedbackCopyright 2006,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedanditssubsidi aries(TI)

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