GroundingofMixedSignalPCBs

www.cnshu.cn中国最大的资料库下载GroundingofMixedSignalPCBsAquestionthatIhearoftenis:HowdoIpreventdigitallogicgroundcurrentsfromcontaminatingmylowlevelanalogcircuitry?Thisisagoodquestionwithoutasimpleanswer.MostA/Dconvertermanufacturer'sdatabooksandapplicationnotesprovidelittleifanyusefulinformationonthesubject.Iftheydoprovideinformation,itisusuallyonlyapplicabletoasimplesystemcontainingonlyoneA/Dconverter.Somepeoplesuggestsplittingthegroundplaneinordertoisolatethedigitalgroundcurrentsfromtheanaloggroundcurrents.Althoughthesplitplaneapproachcanbemadetowork,ithasmanypotentialproblemsespeciallyinlargecomplicatedsystems.Canyoulistsomeoftheseproblems?Oneofthemajoronesisthatyoucannotrouteatraceoverthesplitintheplane(seeTechTipSlotsinGroundPlanes).Itisalwaysbettertohaveonlyasinglereferenceplaneforasystem.Ifyoudosplitthegroundplaneandruntracesacrossthesplit(lefthandfigurebelow),therewillbenoreturnpathnearthetraceandthecurrentwillhavetoflowinabigloop.Currentflowinginbigloopsproduceradiationandhighgroundinductance.Ifyoumustsplitthegroundplaneandruntracesacrossthesplit,youshoulddoitasshownintherighthandfigurebelow.Byconnectingtheplanestogetheratonepoint(abridge)androutingallthetracessothattheycrossatthisbridgepoint,youwillhaveprovidedareturnpathforthecurrentdirectlyunderneatheachofthetraces(henceaverysmalllooparea).Highfrequencydigitalgroundreturncurrentswanttoreturndirectlyunderneaththesignaltrace.Thisisthelowestimpedance(lowestinductance)path.Thedigitalgroundcurrentshavenodesiretoflowthroughtheanalogportionofthegroundplaneandcorruptyouranalogsignal.Whythendoweneedtosplitthegroundtopreventthedigitalcurrentfromdoingsomethingthatitdoesnotwanttodoanyhow?Therefore,IprefertheapproachofusingonlyonegroundplaneandpartitioningthePCBintodigitalandanalogroutingsections.Analogsignalscanthenberoutedonlyintheanalogsectionoftheboard(onanylayer),anddigitalsignalscanberoutedonlyinthedigitalsectionoftheboard(onanylayer).Whatcausesproblemsiswhenadigitalsignalisroutedintheanalogsectionoftheboard,orvisaversa.APCBwithasinglegroundplane,partitionedintoanaloganddigitalsections,anddisciplineinroutingthesignalscanusuallysolveanwww.cnshu.cn中国最大的资料库下载otherwisedifficultlayoutproblem,withoutcreatingtheadditionalproblemscausedbyasplitgroundplane.Ifthelayoutisdoneproperly,thedigitalgroundcurrentswillremaininthedigitalsectionoftheboardandwillnotinterferewiththeanalogsignals.Therouting,however,mustbecheckedcarefullytoassurethattheabovementionedroutingrestrictionsareadheredtoonehundredpercent!ThekeytoasuccessfulmixedsignalPCBlayout,therefore,isproperpartitioningandroutingdiscipline,notasplitgroundplane.ManyA/Dconvertermanufacturers,whilesuggestingtheuseofsplitgroundplanes,statethefollowingintheirdatasheetsorapplicationnotes:TheAGNDandDGNDpinsmustbeconnectedtogetherexternallytothesamelowimpedancegroundplanewithminimumleadlength.AnyextraexternalimpedanceintheDGNDconnectionwillcouplemoredigitalnoiseintotheanalogcircuitthroughthestraycapacitanceinternaltotheIC.TheirrecommendationistoconnectboththeAGNDandtheDGNDpinsoftheA/Dconvertertotheanaloggroundplane.Thisapproachhasthepotentialofcreatinganumberofadditionalproblems.Canyoulistsomeoftheseproblems?Whatdoyouconnectthegroundsideofthedigitalpowerdecouplingcapacitorto?Theanalogplaneorthedigitalplane?AmuchbetterwaytosatisfytherequirementofconnectingAGNDandDGNDpinstogetherthroughalowimpedance,andnotcreateadditionalproblemsintheprocess,istouseonlyonegroundplanetobeginwith.Thekeytodeterringtheoptimumboardlayoutistothink,howandwheredothereturncurrentsflow?IfyouarestillskepticalaboutusingasinglegroundplaneonyourmixedsignalboardsIsuggestyoudothefollowingexperiment.Layouttheboardwithasplitgroundplane,butprovidemeansforconnectingthetwoplanestogetherevery1/2inchwithjumpersorzeroohmresistors.Routetheboardproperly,withnodigitaltraces(onanylayer)overtheanalogplaneandnoanalogtraces(onanylayer)overthedigitalplane.Buildtheboardandtestit'sfunctionalityandEMCperformance.ConnecttheplanestogetherandtesttheboardagainforfunctionalityandEMCperformance.Ithinkthatyouwillfindthatinalmostallcases,boththefunctionalperformanceandtheEMCperformanceoftheboardwillbebetterwiththesinglegroundplane.Ifyoudotheexperimentsendmeane-maillettingmeknowofyourresults.Itisalmostalwaysbettertohaveonlyasinglereferenceplaneforasystem!Summary:www.cnshu.cn中国最大的资料库下载PartitionyourPCBwithseparateanaloganddigitalsections.Donotsplitthegroundplane.Useonesolidgroundplaneunderbothanaloganddigitalsectionsoftheboard.Routedigitalsignalsonlyinthedigitalsectionoftheboard.Thisappliestoalllayers.Routeanalogsignalsonlyintheanalogsectionoftheboard.Thisappliestoalllayers.ThekeytoasuccessfulPCBlayoutistheuseofroutingdiscipline.