运算放大器电路固有噪声的分析与测量2

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1、AnalysisAndMeasurementOfIntrinsicNoiseInOpAmpCircuitsPartII:IntroductionToOpAmpNoisebyArtKay,SeniorApplicationsEngineer,TexasInstrumentsIncorporatedAnimportantcharacteristicofnoiseisitsspectraldensity.Voltagenoisespectraldensityisameasurementofroot-mean-square(rms)noisevoltagepersquarerootHertz

2、(orcommonly:nV/√Hz).PowerspectraldensityisgiveninW/Hz.InthepreviousarticlewelearnedthatthethermalnoiseofaresistorcanbecomputedusingEquation2.1.Thisequationcanberearrangedintoaspectraldensityform.Oneimportantcharacteristicofthisnoiseisthatithasaflatspectraldensityplot(ieithasuniformenergyatallfr

3、equencies).Forthisreason,thermalnoiseissometimescalledbroadbandnoise.Opampsalsohavebroadbandnoiseassociatedwiththem.Broadbandnoiseisdefinedasnoisethathasaflatspectraldensityplot.Eq2.1Fig.2.1:OpAmpNoiseSpectralDensityInadditiontobroadbandnoise,opampsoftenhavealow-frequencynoiseregionthatdoesnoth

4、aveaflatspectraldensityplot.Thisnoiseiscalled1/fnoise,flickernoise,orlow-frequencynoise.Typically,thepowerspectrumof1/fnoisefallsatarateof1/f.This(½)meansthatthevoltagespectrumfallsatarateof1/f.Inpractice,however,theexponentofthe1/ffunctionmaydeviateslightly.Fig.2.1showsatypicalopampspectrumwit

5、hbotha1/fregionandabroadbandregion.Notethatthespectraldensityplotalsoshowscurrentnoise(giveninfA/√Hz).Itisimportanttonotethat1/fnoisealsohasanormaldistributionand,consequently,themathematicsdescribedinPartIstillapply.Fig.2.2showsthetimedomaindescriptionof1/fnoise.Noticethatthex-axisofthisgraphi

6、sgiveninseconds;thisslowchangewithtimeistypicalfor1/fnoise.Fig.2.2:1/fNoiseShownInTheTimeDomainAndStatisticallyThestandardmodelforopampnoiseisshowninFig.2.3.Itconsistsoftwouncorrelatedcurrentnoisesourcesandonevoltagenoisesourceconnectedtotheopampinputs.Thevoltagenoisesourcecanbethoughtofastime-

7、varyinginputoffsetvoltagecomponent,andthecurrentnoisesourcescanbethoughtoftime-varyingbiascurrentcomponents.Fig.2.3:OpAmpNoiseModelOpAmpNoiseAnalysisTechniqueThegoalofopampnoiseanalysistechniqueistocalculatethepeak-to-peakoutputno