SAR-ADC-教程之四

1PresentedbyFrankOhnhäuser,TexasInstrumentsTraining11:DesignAspects:SARADCI2Training111.Designa3bitCDACwiththefollowingconditions:Thesupplyvoltageis5V.Thefullydifferentialinputvoltagerangeshouldbe+/-2.5V,thecommonmoderange1.25Vto3.75V.Thecomparatorinputpotentials(Vcp,Vcn)arelimitedto0V..3.2V.a)Drawthepossibleinputvoltageranges.b)Whichcapacitorswillbeusedforsampling?c)Whatistheidealworkingpointatthecomparatorinputandhowcanitbeimplemented?d)SuggesttheswitchpositionduringtheMSBdecision.e)Calculatethecomparatorinputvoltage.3Training111.Designa3bitCDACwiththefollowingconditions:Thesupplyvoltageis5V.Thefullydifferentialinputvoltagerangeshouldbe+/-2.5V,thecommonmoderange1.25Vto3.75V.Thecomparatorinputpotentials(Vcp,Vcn)arelimitedto0V..3.2V.f)SuggesttheswitchpositionsduringtheBit2decisiondependentontheMSBanddrawtheCDACforthecaseofMSB=1.g)Calculatethecomparatorinputvoltage.h)CalculatethedifferentialinputvoltagerangeoftheADCandtherequiredreferencevoltageREF.i)Confirmthemaximuminputpotentialatthecomparator.j)Finalizetheswitchpositions,ifBit2=04Training112.Designacircuitry,sothattheswitchestoREFand0Vareneverturnedontogether.5Training11a)Drawthepossibleinputvoltageranges.VtFullyDifferentialSignal–CMV=1.25VINPINNPFSNFS2.5V0VVtFullyDifferentialSignal–CMV=3.75VINPINNPFSNFS5V2.5V6Training11b)Whichcapacitorswillbeusedforsampling:Vinp,n=0..5VVcp,n=0..3.2VTheinputsignalneedtobedividedbymorethanafactorof1.6.→SampleonMSBonly.7Training11c)Whatistheidealworkingpointatthecomparatorinputandhowcanitbeimplemented?Vcp,n=0..3.2V→Choosethepotentialatthecomparatorinputduringsamplingaround1.6V8Training11d)SuggesttheswitchpositionduringtheMSBdecision.BipolarADC→DuringtheMSBdecision,thecomparatordecides,ifthedifferentialinputvoltageissmallerorhigherthan0V.A0Vinputvoltageneedstoresultina0VcomparatorvoltageduringtheMSBdecision.→ItmustbepossibletoincreaseordecreasetheCDACoutputvoltagebythesameamountdependingontheMSBdecision.9Training11d)SuggesttheswitchpositionduringtheMSBdecision.10Training11)V6.1V0(C)V6.1V(CQn,inpn,sp−⋅+−⋅=)VV0(C)VREF(CQ)VV0(C)VREF(CQcncnn,MSBcpcpp,MSB−+−⋅=−+−⋅=inninpinpinncncpcVVV02VVVVV−=−=2VREFV6.1V2VREFV6.1Vinncninpcp−+=−+=e)Calculatethecomparatorinputvoltage11DACf)SuggesttheswitchpositionsduringtheBit2decisiondependentontheMSBanddrawtheCDACforthecaseofMSB=1MSB=1→Vc0V→increaseVcp→leaveCatREFandswitchC/2toREFMSB=0→Vc0V→decreaseVcp→switchCto0VandC/2toREF12Training11)VV0(C)VREF(CQ)V(C5.0)VREF(C5.1Qcncnn,2cpcpp,2−+−⋅=−+−⋅=2REFVVV0VV2REFV2VREFV6.1V,2VREF5.1V6.1Vinninpinninpcinncpinpcp−⇒+−=−+=−⋅+=f)SuggesttheswitchpositionsduringtheBit2decisiondependentontheMSBanddrawtheCDACforthecaseofMSB=1g)Calculatethecomparatorinputvoltage.13Training11h)CalculatethedifferentialinputvoltagerangeoftheADCandtherequiredreferencevoltageREF.SwitchingC/2changesthecomparatordecisionfromVinp-Vinn0VtoVinp-VinnREF/2→ConnectingallcapacitorstoREFwouldresultinVinp-VinnREF→Connectingallcapacitorsto0VwouldresultinVinp-Vinn-REF→-REFVinp-VinnREF,REF=2.5V14Training11V0V35.02V5V5.2V6.1VV2.3V85.22V0V5.2V6.1V2VREFV6.1Vmin,n,cpmax,n,cpn,inpn,cp=−+==−+=−+=i)Confirmthemaximuminputpotentialatthecomparator.ThehighestandlowestcomparatorpotentialscanoccurduringtheMSBdecision.15Training11)VV0(C)VREF(CQ)V(C75.0)VREF(C25.1Qcncnn,3cpcpp,3−+−⋅=−+−⋅=4REFVV02VVREF25.0V2VREFV6.1V,2VREF25.1V6.1Vinninpinninpcinncninpcp+⇒+−=−+=−+=j)Finalizetheswitchpositions,ifBit2=016Training112.Designacircuitry,sothattheswitchestoREFand0Vareneverturnedontogether.