布线2012【英文版】

Layout/Grounding(Precision,HighSpeedandDigital),Inthischapter,wewilldiscussfivetopics.Thefirsttopiccoversfundamentalsimilaritiesanddifferencesbetweenanaloganddigitallayout.Thenwewilltalkaboutthehiddencomponents(resistors,inductors,andcapacitors)embeddedinyourPCboard.ThenextsectionofthischapterwilltalkabouthowtoimproveyourA/Dconverteraccuracyandresolution.Gettinganotherconverterwillnothelphere.FocusingontheinteractionbetweenthePCBandyourconverterwillimproveyourresults.Thiswillbefollowedbythefourthsectionwherewewilldiscusstwolayerlayouttechniques.Finally,wewillendwithanexampleofhowtodoapoorlayoutandthenhowtofixit.,TheSimilaritiesofAnalogandDigitalLayoutPractices,BypassorDecouplingCapacitorsThePowerandGroundShouldBeRoutedTogether,TheSimilaritiesofAnalogandDigitalLayoutPractices,BypassorDecouplingCapacitorsIntermsoflayout,analogdevicesanddigitaldevicesallrequirethesetypesofcapacitors.Bothtypesofdevicesrequirethatyoupositiononecapacitorasclosetothepowersupplypin(s)aspossible.Acommonvalueforthiscapacitoris0.1F,butitisnotunusualtofinda1Fbypasscapacitor(forlowerfrequencycircuits)ora0.01Fcapacitorinhigherfrequencycircuits.TheselectionofthepropercapacitorforyourcircuitisdiscussedindetailinChapter6.Asecondclassofbypassordecouplingcapacitorinthesystemisrequiredatthepowersupplysource.Thevalueofthiscapacitorisusuallyabout10F.,TheSimilaritiesofAnalogandDigitalLayoutPractices,BypassorDecouplingCapacitorsFigure11.1:InanaloganddigitalPCBdesign,youshouldplacethebypassordecouplecapacitors(0.1F)asclosetothedeviceaspossible.Youshouldalsoplacethepowersupplydecoupling-capacitor(10F)atthepower-sourceorwherethepower-busenterstheboard.Inallcases,thesecapacitorsshouldhaveshortleads.,TheSimilaritiesofAnalogandDigitalLayoutPractices,BypassorDecouplingCapacitorsTheplacementofthebypassordecouplingcapacitorsarejustcommonsenseforbothtypesofdesigns,butinterestingenough,fordifferentreasons.Intheanaloglayoutdesign,bypasscapacitorsgenerallyservethepurposeofredirectinghighfrequencysignalsonthepowersupplytrace.Thisnoisewouldotherwiseenterintothesensitiveanalogchip,throughthepowersupplypin.Generally,thesehighfrequencysignalsoccuratfrequenciesbeyondrejectioncapabilityoftheanalogdevice.Thepossibleconsequencesofnotusingabypasscapacitorinyouranalogcircuitresultsintheadditionofunduenoisetothesignalpathorworseyet,oscillation,TheSimilaritiesofAnalogandDigitalLayoutPractices,BypassorDecouplingCapacitorsFordigitaldevices,suchascontrollersandprocessors,thedecouplingcapacitoronthepowersupplypinarerequired,butforadifferentreason.Oneofthefunctionsofthesecapacitorsservesasa“mini”chargereservoir.Frequentlyindigitalcircuits,agreatdealofcurrentisrequiredtoexecutethetransitionsofthechanginggatestates.Becauseoftheswitchingtransientcurrentsthatoccuronthechipandthroughoutthecircuitboard,havingadditionalcharge“on-call”isadvantageous.Theconsequenceofnothavingenoughchargelocallytoexecutethisswitchingactioncouldresultinasignificantdynamicandstaticchangeinthepowersupplyvoltage.Whenthevoltagechangeistoolarge,itwillcausethedigitalsignalleveltogointotheindeterminatestate.,TheSimilaritiesofAnalogandDigitalLayoutPractices,BypassorDecouplingCapacitorsSoformultiplereasons,itisagoodideatobypass(ordecouple)thepowersupplyatthepowersupplyandatthepowersupplypinofalloftheactivedevices.,TheSimilaritiesofAnalogandDigitalLayoutPractices,2.ThePowerandGroundShouldBeRoutedTogetherWhenyoumatchpowerandgroundtraceswithrespecttolocation,youlessontheopportunitiesforEMI.Ifyoudontmatchpowerandground,systemloopsarepartofthelayout.Thepossibilityofseeing“noisy“resultswithoutexplanationisreal.Figure11.2showsanexampleofaPCBdesignwiththeunmatchedpowerandgroundtraces.,TheSimilaritiesofAnalogandDigitalLayoutPractices,2.ThePowerandGroundShouldBeRoutedTogetherWhenyoumatchpowerandgroundtraceswithrespecttolocation,youlessontheopportunitiesforEMI.Ifyoudontmatchpowerandground,systemloopsarepartofthelayout.Thepossibilityofseeing“noisy“resultswithoutexplanationisreal.Figure11.2showsanexampleofaPCBdesignwiththeunmatchedpowerandgroundtraces.,TheSimilaritiesofAnalogandDigitalLayoutPractices,2.ThePowerandGroundShouldBeRoutedTogether,Figure11.2:Thepowerandgroundtracesarelaidoutusingdifferentroutestothedeviceonthisboard.ThismismatchopenstheopportunityforEMIintotheelectronicsofthisboard.,TheloopareainFigure11.2is697cm2.Thisloopisaperfectantennafornoiseinthearea.Withthisboard,youmaybeabletopickupradiosignals.Inthe1980s,oneoftheGermanengineersthatIworkedwithwasabletodesignboardsofthisclassand“pick-up”radio-freeEurope.,TheSimilaritiesofAnalogandDigitalLayoutPractices,2.ThePowerandGroundShouldBeRoutedTogether,Figure11.3:Inthisonelayerboard,thepowertraceandgroundtracearelaidnexttoeachotherontheirwaytothedeviceonthisboard.Figure11.2showsaboardwherethetracesarebettermatched.TheopportunityforEMIintotheelectronicsofthisboardislessenedby679/12.8or54x.,InFigure11.3,thesignalandgroundlinearenexttoeachother.Thisgreatlyreducesthelooparea.Anevenbettersolutionwouldbetohaveagroundplane,whichwouldbeunderneaththepowersupplytrace.Anevenbettersolutionwouldbetohaveagroundplaneandaseparatepowerplane.,概貌图,注意单片机电源和地线的环路高亮部分为GND,注意单片机电源和地线的环路高亮部分为Vcc,TheSimilaritiesofAnalogandDigitalLayoutPractices,Thefundamentalsofcircuitboardlayoutapplytoanalogcircuitsaswellasdigitalcircuits.Onefundamentalruleofthumbistouseuninterruptedgroundplanes.ThiscommonpracticereducestheeffectsofI/t(changeincurrentwithtime)indigitalcircuits.Indigitalcircuits,thechangeincurrentwithtimechangesthepotentialofground.Inanalogcircuits,injectednoiseiscausedbyI/t.But,whencomparingdigitalandanalogcircuits,youshouldexerciseanaddedprecautionwithanalogcircuitsinordertokeepthedigitalsignallinesandreturnpathsinthegroundplaneasfarawayfromtheanalogcircuitryaspossible.,3.WheretheDomainsDifferGroundPlanesCanBeaProblem,TheSimilaritiesofAnalogandDigitalLayoutPractices,Thiscanbedonebyconnectingtheanaloggroundplaneseparatelytothesystemgroundorhavingtheanalogcircuitryatthefarthestsideoftheboardthatis,attheendoftheline.Thisisdonesothatsignalpathshaveaminimalamountofinterferencefromexternalsources.Theoppositeisnottruefordigitalcircuitry.Thedigitalcircuitrycantolerateagreatdealofnoiseonthegroundplanebeforeproblemsstarttoappear.,3.WheretheDomainsDifferGroundPlanesCanBeaProblem,TheSimilaritiesofAnalogandDigitalLayoutPractices,LocationofComponentsIneveryPCBdesign,youshouldseparatethenoisyandquietportionsofthecircuit,asmentionedabove.Generally,thedigitalcircuitryis“rich”withnoise.Alternatively,digitalcircuitryislesssensitivetothistypeofnoisebecauseofthelargervoltagenoisemargins.,3.WheretheDomainsDifferGroundPlanesCanBeaProblem,LocationofComponentsWhenyoulookatanalogcircuits,youwilleasilyfindthattheyarenotasforgivingasthedigitalcircuits.Thevoltagenoisemarginsoftheanalogcircuitryaremuchsmaller.Ofthetwodomains,theanalogdomainismostsensitivetoswitchingnoise.Inthelayoutofamixedsignalsystem,youshouldseparatethetwodomains.Figure11.4showsthisisgraphically.,TheSimilaritiesofAnalogandDigitalLayoutPractices,3.WheretheDomainsDifferGroundPlanesCanBeaProblem,TheSimilaritiesofAnalogandDigitalLayoutPractices,3.WheretheDomainsDifferGroundPlanesCanBeaProblem,Figure11.4:Ifpossible,(a)thedigitalandanalogportionofcircuitsshouldbeseparatedinordertoseparatethedigitalswitchingactivityfromtheanalogcircuitry.Additionally,(b)thehighfrequencyshouldbeseparatedfromthelowfrequencywherepossible,keepingthehigherfrequencycomponentsclosertotheboardconnector.,4.WheretheBoardandComponentParasiticsCanDotheMostDamageThemajorclassesofparasiticsgeneratedbythePCboardlayoutcomeintheformofresistors,capacitors,andinductors.Forinstance,youcanbuildPCBresistorswithyourtracesthatspanbetweencomponents.Youcanbuildunintentionalcapacitorsintotheboardwithtraces,solderingpads,andparalleltraces.,TheSimilaritiesofAnalogandDigitalLayoutPractices,4.WheretheBoardandComponentParasiticsCanDotheMostDamageUnintentionalinductorscomefromloopinductance,mutualinductance,andvias.AlloftheseparasiticsstandachanceofinterferingwiththeeffectivenessofyourcircuitasyoutransitionfromthecircuitdiagramtotheactualPCB.Youwillclearlyseeinthissectionofthechapterthemosttroublesomeclassofboardparasiticsandseeexamplesofwheretheseparasiticstheeffectoncircuitperformance.,TheSimilaritiesofAnalogandDigitalLayoutPractices,4.WheretheBoardandComponentParasiticsCanDotheMostDamageFeelingthePainofThoseUnnecessaryCapacitors,TheSimilaritiesofAnalogandDigitalLayoutPractices,Figure11.5:YoucaneasilyplacecapacitorsintoaPCBbylayingouttwotracesincloseproximity.Withthistypeofcapacitor,fastvoltagechangesononetracecaninitiateacurrentsignalintheothertrace.(Chapter9alsodiscussesthisissue.),4.WheretheBoardandComponentParasiticsCanDotheMostDamageYoucaneasilyminimizethistypeofphenomenausingoneoftwotechniques(1)(2)Anothertechniqueusedistolayagroundtracebetweenthetwooffendingtraces.Notonlyisthegroundtracelowimpedance,butanadditionaltracelikethiswillbreakuptheE-fieldsthatarecausingthedisturbance.,TheSimilaritiesofAnalogandDigitalLayoutPractices,4.WheretheBoardandComponentParasiticsCanDotheMostDamageThistypeofcapacitorcancauseproblemsinmixedsignalcircuitswheresensitive,highimpedance,analogtracesareincloseproximitytodigitaltraces.,TheSimilaritiesofAnalogandDigitalLayoutPractices,Example:,4.WheretheBoardandComponentParasiticsCanDotheMostDamage,TheSimilaritiesofAnalogandDigitalLayoutPractices,Thisconfigurationproducesinconsistentnoiseontheanaloglinebecausethedatainputcodeonthatparticulardigitaltracechanges.Thesechangesaredependentontheprogrammingrequirementsforthedigitalpotentiometer.,Figure11.7:ThisisthefirstattemptatthelayoutforthecircuitinFigure11.6.Inthisfigure,youcanseethatacritical,highimpedance,analoglineisveryclosetoadigitaltrace.,4.WheretheBoardandComponentParasiticsCanDotheMostDamageOnthebench,Imeasuredthedigitalsignalthatwascoupledintothesensitive,analogwiper,trace.Figure11.8showsthescopephoto.,TheSimilaritiesofAnalogandDigitalLayoutPractices,4.WheretheBoardandComponentParasiticsCanDotheMostDamage,TheSimilaritiesofAnalogandDigitalLayoutPractices,Figure11.9:Withanewlayout,theanaloglinesareseparatefromthedigitallines.Thisdistancehaseliminatedthedigitalnoisethatwascausinginterferenceinthepreviouslayout.,4.WheretheBoardandComponentParasiticsCanDotheMostDamage,TheSimilaritiesofAnalogandDigitalLayoutPractices,Onceagain,whenthedigitalandanalogdomainsmeet,carefullayoutiscriticalifyouintendtohaveasuccessfulfinalPCBimplementation.Inparticular,activedigitaltracesclosetohighimpedance,analogtraceswillcauseseriouscouplingnoise.Youcanavoidthisnoisecouplingphenomenabyputtingdistancebetweentraces.,4.WheretheBoardandComponentParasiticsCanDotheMostDamage,TheSimilaritiesofAnalogandDigitalLayoutPractices,5.InductorsDesignedintothePCBThewaythataninductorisdesignedintoaboardissimilartotheconstructionofacapacitor.Againthisisdonebyplacingtwotraces,oneontopoftheotherwithtwolayersorbyplacingthembesideeachotheronthesamelayer,asshowninFigure11.11.,TheSimilaritiesofAnalogandDigitalLayoutPractices,Inbothtraceconfigurations,changesincurrentwithtime(I/t)ononetracecouldgenerateavoltageinthesametraceduetotheinductanceonthattraceandinitiateaproportionalcurrentonthesecondtraceduetothemutualinductance,5.InductorsDesignedintothePCB,TheSimilaritiesofAnalogandDigitalLayoutPractices,ToeliminatepotentialnoiseforEMIsourcesitisbesttoseparatequietanaloglinesversusnoisyI/Oports.Trytoimplementlowimpedancepowerandgroundnetworks,minimizeinductanceinconductorsfordigitalcircuitsandminimizecapacitivecouplinginanalogcircuits.,TheArtofLayingOutTwo-LayerBoards,PayNoworPayLaterwiththeAutoRouterandAnalogCircuits,Gospel:Itistemptingtousetheautorouterwhendesigningprintedcircuitboard(PCB).Moreoftenthannot,apurelydigitalboard,(especiallyifthesignalsarerelativelyslow,andthecircuitdensityislow)willworkjustfine.Butasyoutrytolayoutanalog,mixedsignalorhigh-speedcircuitswiththeautoroutingtoolthatisavailablewithyourlayoutsoftware,theremaybesomeissues.Theprobabilityofcreatingseriouscircuitperformanceproblemsisveryreal.,TheArtofLayingOutTwo-LayerBoards,PayNoworPayLaterwiththeAutoRouterandAnalogCircuits,Withthislayout,thereareseveralareasofconcern,butthemosttroublingissueisthegroundingstrategy.Ifyoufollowthegroundtracesonthetoplayer,thetracesconnecteverydeviceonthatlayer.Asecondgroundconnect