Grounding Data Converters and Solving the Mystery of AGND and DGND.docx

Grounding Data Converters and Solving the Mystery of AGND and DGND.docx

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GroundingDataConvertersandSolvingtheMysteryofAGNDandDGND

GroundingDataConvertersandSolvingtheMysteryof"AGND"and"DGND"INTRODUCTION

Today'ssignalprocessingsystemsgenerallyrequiremixed-signaldevicessuchasanalog-to-digitalconverters（ADCs）anddigital-to-analogconverters（DACs）aswellasfastdigitalsignalprocessors（DSPs）.RequirementsforprocessinganalogsignalshavingwidedynamicrangesincreasestheimportanceofhighperformanceADCsandDACs.Maintainingwidedynamicrangewithlownoiseinhostiledigitalenvironmentsisdependentuponusinggoodhigh-speedcircuitdesigntechniquesincludingpropersignalrouting,decoupling,andgrounding.

Inthepast,"highprecision,low-speed"circuitshavegenerallybeenvieweddifferentlythanso-called"high-speed"circuits.WithrespecttoADCsandDACs,thesampling（orupdate）frequencyhasgenerallybeenusedasthedistinguishingspeedcriteria.However,thefollowingtwoexamplesshowthatinpractice,mostoftoday'ssignalprocessingICsarereally"high-speed,"andmustthereforebetreatedassuchinordertomaintainhighperformance.ThisiscertainlytrueofDSPs,andalsotrueofADCsandDACs.

AllsamplingADCs（ADCswithaninternalsample-and-holdcircuit）suitableforsignalprocessingapplicationsoperatewithrelativelyhighspeedclockswithfastriseandfalltimes（generallyafewnanoseconds）andmustbetreatedashighspeeddevices,eventhoughthroughputratesmayappearlow.Forexample,amedium-speed12-bitsuccessiveapproximation（SAR）ADCmayoperateona10-MHzinternalclock,whilethesamplingrateisonly500kSPS.

Sigma-delta（Σ-Δ）ADCsalsorequirehighspeedclocksbecauseoftheirhighoversamplingratios.Evenhighresolution,so-called"lowfrequency"Σ-ΔindustrialmeasurementADCs（havingthroughputsof10Hzto7.5kHz）operateon5-MHzorhigherclocksandofferresolutionto24-bits（forexample,theAnalogDevicesAD77xx-series）.

Tofurthercomplicatetheissue,mixed-signalICshavebothanaloganddigitalports,andbecauseofthis,muchconfusionhasresultedwithrespecttopropergroundingtechniques.Inaddition,somemixed-signalICshaverelativelylowdigitalcurrents,whileothershavehighdigitalcurrents.Inmanycases,thesetwotypesmustbetreateddifferentlywithrespecttooptimumgrounding.

Digitalandanalogdesignengineerstendtoviewmixed-signaldevicesfromdifferentperspectives,andthepurposeofthistutorialistodevelopageneralgroundingphilosophythatwillworkformostmixedsignaldevices,withouthavingtoknowthespecificdetailsoftheirinternalcircuits.

GROUNDANDPOWERPLANES

Theimportanceofmaintainingalowimpedancelargeareagroundplaneiscriticaltoallanaloganddigitalcircuitstoday.Thegroundplanenotonlyactsasalowimpedancereturnpathfordecouplinghighfrequencycurrents（causedbyfastdigitallogic）butalsominimizesEMI/RFIemissions.Becauseoftheshieldingactionofthegroundplane,thecircuit'ssusceptibilitytoexternalEMI/RFIisalsoreduced.

Groundplanesalsoallowthetransmissionofhighspeeddigitaloranalogsignalsusingtransmissionlinetechniques（microstriporstripline）wherecontrolledimpedancesarerequired.

Theuseof"busswire"istotallyunacceptableasa"ground"becauseofitsimpedanceattheequivalentfrequencyofmostlogictransitions.Forinstance,#22gaugewirehasabout20nH/inchinductance.Atransientcurrenthavingaslewrateof10mA/nscreatedbyalogicsignalwoulddevelopanunwantedvoltagedropof200mVatthisfrequencyflowingthrough1inchofthiswire:

Forasignalhavinga2-Vpeak-to-peakrange,thistranslatesintoanerrorofabout10%（approximately3.5-bitaccuracy）.Eveninall-digitalcircuits,thiserrorwouldresultinconsiderabledegradationoflogicnoisemargins.

Figure1showstheclassicillustrationofasituationwherethedigitalreturncurrentmodulatestheanalogreturncurrent（topfigure）.Thegroundreturnwireinductanceandresistanceissharedbetweentheanaloganddigitalcircuits,andthisiswhatcausestheinteractionandresultingerror.ApossiblesolutionistomakethedigitalreturncurrentpathflowdirectlytotheGNDREFasshowninthebottomfigure.Thisisthefundamentalconceptofa"star,"orsingle-pointgroundsystem.Implementingthetruesingle-pointgroundinasystemwhichcontainsmultiplehighfrequencyreturnpathsisdifficultbecausethephysicallengthoftheindividualreturncurrentwireswillintroduceparasiticresistanceandinductancewhichcanmakeobtainingalowimpedancehighfrequencygrounddifficult.Inpractice,thecurrentreturnsmustconsistoflargeareagroundplanesforlowimpedancetohighfrequencycurrents.Withoutalow-impedancegroundplane,itisthereforealmostimpossibletoavoidthesesharedimpedances,especiallyathighfrequencies.

Figure1:

DigitalCurrentsFlowinginAnalogReturnPathCreateErrorVoltages

Allintegratedcircuitgroundpinsshouldbesoldereddirectlytothelow-impedancegroundplanetominimizeseriesinductanceandresistance.TheuseoftraditionalICsocketsisnotrecommendedwithhigh-speeddevices.Theextrainductanceandcapacitanceofeven"lowprofile"socketsmaycorruptthedeviceperformancebyintroducingunwantedsharedpaths.IfsocketsmustbeusedwithDIPpackages,asinprototyping,individual"pinsockets"or"cagejacks"maybeacceptable.Bothcappedanduncappedversionsofthesepinsocketsareavailable（AMPpartnumbers5-330808-3,and5-330808-6）.Theyhavespring-loadedgoldcontactswhichmakegoodelectricalandmechanicalconnectiontotheICpins.Multipleinsertions,however,maydegradetheirperformance.

LOWANDHIGHFREQUENCYDECOUPLING

Eachpowersupplyshouldbedecoupledtothelow-impedancegroundplanewithahighqualityelectrolyticcapacitoratthepointitentersthePCboard.Thisminimizeslowfrequencynoiseonthesupplyruns.Ateachindividualanalogstage,furtherlocal,high-frequency-onlyfilteringisrequiredattheindividualICpackagepowerpins.

Figure2showsthistechnique,inbothcorrect（left）aswellasincorrectexampleimplementations（right）.Intheleftexample,atypical0.1-µFchipceramiccapacitorgoesdirectlytotheoppositePCBsidegroundplane,byvirtueofthevia,andontotheIC'sGNDpinbyasecondvia.Incontrast,thelessdesirablesetupattherightaddsadditionalPCBtraceinductanceinthegroundpathofthedecouplingcap,reducingeffectiveness.

Figure2:

LocalizedHighFrequencySupplyFilter（s）ProvidesOptimumFilteringandDecouplingViaShortLow-InductancePath（GroundPlane）

Allhighfrequency（i.e.,≥10MHz）ICsshoulduseabypassingschemesimilartoFigure2forbestperformance.Theferritebeadsaren't100%necessary,buttheywilladdextrahighfrequencynoiseisolationanddecoupling,whichisoftendesirable.Possiblecaveatsherewouldbetoverifythatthebeadsneversaturate,whentheICsarehandlinghighcurrents.

Notethatwithsomeferrites,evenbeforefullsaturationoccurs,somebeadscanbenon-linear,soifapowerstageisrequiredtooperatewithalowdistortionoutput,thisshouldalsobechecked.

DOUBLE-SIDEDVS.MULTILAYERPRINTEDCIRCUITBOARDS

EachPCBinthesystemshouldhaveatleastonecompletelayerdedicatedtothegroundplane.Ideally,adouble-sidedboardshouldhaveonesidecompletelydedicatedtogroundandtheothersideforinterconnections.Inpractice,thisisnotpossible,sincesomeofthegroundplanewillcertainlyhavetoberemovedtoallowforsignalandpowercrossovers,vias,andthrough-holes.Nevertheless,asmuchareaaspossibleshouldbepreserved,andatleast75%shouldremain.Aftercompletinganinitiallayout,thegroundlayershouldbecheckedcarefullytomakesuretherearenoisolatedground"islands,"becauseICgroundpinslocatedinaground"island"havenocurrentreturnpathtothegroundplane.Also,thegroundplaneshouldbecheckedfor"skinny"connectionsbetweenadjacentlargeareaswhichmaysignificantlyreducetheeffectivenessofthegroundplane.Needlesstosay,auto-routingboardlayouttechniqueswillgenerallyleadtoalayoutdisasteronamixed-signalboard,somanualinterventionishighlyrecommended.

SystemsthataredenselypackedwithsurfacemountICswillhavealargenumberofinterconnections;thereforemultilayerboardsaremandatory.Thisallowsatleastonecompletelayertobededicatedtoground.Asimple4-layerboardwouldhaveinternalgroundandpowerplanelayerswiththeoutertwolayersusedforinterconnectionsbetweenthesurfacemountcomponents.Placingthepowerandgroundplanesadjacenttoeachotherprovidesadditionalinter-planecapacitancewhichhelpshighfrequencydecouplingofthepowersupply.Inmostsystems,4-layersarenotenough,andadditionallayersarerequiredforroutingsignalsaswellaspower.

MULTICARDMIXED-SIGNALSSYSTEMS

Thebestwayofminimizinggroundimpedanceinamulticardsystemistousea"motherboard"PCBasabackplaneforinterconnectionsbetweencards,thusprovidingacontinuousgroundplanetothebackplane.ThePCBconnectorshouldhaveatleast30-40%ofitspinsdevotedtoground,andthesepinsshouldbeconnectedtothegroundplaneonthebackplanemotherboard.Tocompletetheoverallsystemgroundingschemetherearetwopossibilities:

1.Thebackplanegroundplanecanbeconnectedtochassisgroundatnumerouspoints,therebydiffusingthevariousgroundcurrentreturnpaths.Thisiscommonlyreferredtoasa"multipoint"groundingsystemandisshowninFigure3.

2.Thegroundplanecanbeconnectedtoasinglesystem"starground"point（generallyatthepowersupply）.

Thefirstapproachismostoftenusedi