第1页共3页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第1页共3页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).第2页共3页第1页共3页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第2页共3页Whatcausesproblemsiswhenadigitalsignalisroutedintheanalogsectionoftheboard,orvisaversa.APCBwithasinglegroundplane,partitionedintoanaloganddigitalsections,anddisciplineinroutingthesignalscanusuallysolveanotherwisedifficultlayoutproblem,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(on第3页共3页第2页共3页编号：时间：2021年x月x日书山有路勤为径，学海无涯苦作舟页码：第3页共3页anylayer)overthedigitalplane.Buildtheboardandtestit'sfunctionalityandEMCperformance.ConnecttheplanestogetherandtesttheboardagainforfunctionalityandEMCperformance.Ithinkthatyouwillfindthatinalmostallcases,boththefunctionalperformanceandtheEMCperformanceoftheboardwillbebetterwiththesinglegroundplane.Ifyoudotheexperimentsendmeane-maillettingmeknowofyourresults.Itisalmostalwaysbettertohaveonlyasinglereferenceplaneforasystem!Summary:PartitionyourPCBwithseparateanaloganddigitalsections.Donotsplitthegroundplane.Useonesolidgroundplaneunderbothanaloganddigitalsectionsoftheboard.Routedigitalsignalsonlyinthedigitalsectionoftheboard.Thisappliestoalllayers.Routeanalogsignalsonlyintheanalogsectionoftheboard.Thisappliestoalllayers.ThekeytoasuccessfulPCBlayoutistheuseofroutingdiscipline.
