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1、HighspeeddigitaldesignNO.4Deng.money@inventec.com3/22/2003GroundandpowerintroductionGroundandpowerplanesinhighspeedsystemsperformthreecriticalfunctions:ProvidestablereferencevoltageforexchangingdigitalsignalsDistributepowertoalllogicdevicesControlcrosstalkbetween

2、signalsPurposesofPowersystem1.Providestablevoltagereferencesforexchangingdigitalsignals2.Distributepowertoalllogicdevices.Referencevoltagegeneration(1)Inputgateuseadifferentialamplifier,comparingtheincomingvoltageagainstitsinternalreferencevoltage.Theinternalrefe

3、renceusuallyconnectstosomecombinationofthepowerinputterminals.Referencevoltagegeneration(2)AnynoiseinducesadifferencebetweenthegroundterminalsofgatesAandC.NoiseNreducesthenoisemarginforgateC.Themostcommoncauseisreturningsignalcurrents.WhengateAsendasignaltogateC,

4、thereturncurrentwillgenerateavoltageatthispointbecozoftheimpedanceofgroundwire.Powerrule1Uselow-impedancegroundconnectionsbetweengatesThisavoidproblemswithcommon-pathnoiseAsolidgroundplanepresentsaremarkablylowinductancetoreturningcurrent.Referencevoltagegenerati

5、on(3)Common-pathnoiseisrelatedtothemutualinductivecouplingdescribedbefore.Lowgroundinductancealonedoesnotsolvethecommon-pathnoiseproblem.Common-pathinductanceinthepowerwiringcanstillcausetrouble.Referencevoltagegeneration(4)IntheHIstate,agate’soutputvoltagedepend

6、sonthevoltageatitspowerterminal,anychangesinthepowervoltage,causedbyreturningsignalcurrentsflowinginthepowerwiring,directlyaffecttheoutputvoltage.Powerrule2Theimpedancebetweenpowerpinsonanytwogatesshouldbejustaslowastheimpedancebetweengroundpins.Referencevoltageg

7、eneration(5)Tomaintainstabletransmittedsignallevels,theimpedanceofthepowersupplymustbeverylow.aswellastheimpedanceofboththegroundandpowerconnections.Theonlypathbetweenpowerandgroundisthroughthebattery.Referencevoltagegeneration(6)Inapracticalpowersystemdesign,the

8、reareothercomponentswhichprovidethislow-impedancepath.Howeveritisaccomplished,theremustbesomelow-impedancepathbetweenpowerandground.Powerrule3Theremustbealow-