非常详细的阻抗匹配设计.pdf

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1、TraceTerminationPWBtraces[orcables]shouldbeterminated(usingoneoftheschemeslistedbelow)whenthetracelengthexceedsthefollowing:Length>tr/[2xtpr].Wheretr=Signalrisetime,tpr=Signalpropagationrate.Forageneralapproximationthispageuses:150ps/inchforFR4[BoardMaterial],and130pS/inchforPolimide[BoardMater

2、ial].Forexample,usingFR4[150ps/inch]atracewitha1.1nSrisetimewouldneedtobeterminatedifitexceeded3.3inches.Thefourmainwaystoterminateasignaltraceareshownbelow.CalculationsforSignalpropagationrate[byboardtype],andreflectionamplitudeandfrequencyareshownaftertheterminationexamples.Asasidenote:APrint

3、edWiringBoard[PWB]tracereallyhasnoresistance[becauseit'sverylow],thispagedealswithPrintedWiringBoardtraceimpedance.TheresistanceofaPrintedWiringBoardtracehasmoretodowithvoltagedropoverthesignalline[trace]andnothingtodowithsignalreflections~whichthispagedealswith.ThispageusesthetermsPrintedWirin

4、gBoard,orPWB,andPrintedCircuitCard,orPCC,andCircuitCardAssembly,CCAInterchangeably.UnusedICinputpinswhichrequireaResistorpull-uparenotdiscussedonthispage.Alsotheinformationprovidedworksforterminatingacableinadditiontoaboardtrace.SeriesTraceTerminationSeries[Source]Terminationshouldonlybeusedwit

5、honeloadontheline,butonlyrequiresoneresistor[R],placednearthesource.SourceTerminationalsoworkswellwhenthedriverimpedanceislessthanthecharacteristicimpedanceoftheline.ItalsocausesnoDCpath.TheSeriesTerminationresistor[R]shouldbeselectedsothatthecombinationoftheresistor[R]andtheoutputresistance[Zs

6、]ofthedrivermatchesthetraceimpedance[Zo].SourceTerminationmayeffecttherisetimeofthesignalbecauseoftheRCtimeconstant[Rofresistor,Cofthecable].Thedriverisspecifiedwitharisetimebasedonsomeunitload[say10pF],therisetimewillbereducetotheR*Ctimeconstantwiththeseriesresistor.Astherisetimeisdecreasedthe

7、overallcurrentdemandfromthedrivingpinisalsoreduced.Reducingtherisetimehasonebenefit,reducingtheInstantaneouscurrentdemandonthedriver[reducinggroundbounceandEMI]Aseriesterminatedlinedoesnotstopreflections[theloadisstillnotterminate