高速PCB布局的实际指南

AnalogDialogue39-09,September(2005)1APracticalGuidetoHigh-SpeedPrinted-Circuit-BoardLayoutByJohnArdizzoni[[e-mail]]Despiteitscriticalnatureinhigh-speedcircuitry,printed-circuit-board(PCB)layoutisoftenoneofthelaststepsinthedesignprocess.Therearemanyaspectstohigh-speedPCBlayout;volumeshavebeenwrittenonthesubject.Thisarticleaddresseshigh-speedlayoutfromapracticalperspective.Amajoraimistohelpsensitizenewcomerstothemanyandvariousconsiderationstheyneedtoaddresswhendesigningboardlayoutsforhigh-speedcircuitry.Butitisalsointendedasarefreshertobenefitthosewhohavebeenawayfromboardlayoutforawhile.Noteverytopiccanbecoveredindetailinthespaceavailablehere,butweaddresskeyareasthatcanhavethegreatestpayoffinimprovingcircuitperformance,reducingdesigntime,andminimizingtime-consumingrevisions.Althoughthefocusisoncircuitsinvolvinghigh-speedopamps,thetopicsandtechniquesdiscussedherearegenerallyapplicabletolayoutofmostotherhigh-speedanalogcircuits.WhenopampsoperateathighRFfrequencies,circuitperformanceisheavilydependentontheboardlayout.Ahigh-performancecircuitdesignthatlooksgood“onpaper”canrendermediocreperformancewhenhamperedbyacarelessorsloppylayout.Thinkingaheadandpayingattentiontosalientdetailsthroughoutthelayoutprocesswillhelpensurethatthecircuitperformsasexpected.TheSchematicAlthoughthereisnoguarantee,agoodlayoutstartswithagoodschematic.Bethoughtfulandgenerouswhendrawingaschematic,andthinkaboutsignalflowthroughthecircuit.Aschematicthathasanaturalandsteadyflowfromlefttorightwilltendtohaveagoodflowontheboardaswell.Putasmuchusefulinformationontheschematicaspossible.Thedesigners,technicians,andengineerswhowillworkonthisjobwillbemostappreciative,includingus;attimesweareaskedbycustomerstohelpwithacircuitbecausethedesignerisnolongerthere.Whatkindofinformationbelongsonaschematicbesidestheusualreferencedesignators,powerdissipations,andtolerances?Hereareafewsuggestionsthatcanturnanordinaryschematicintoasuperschematic!Addwaveforms,mechanicalinformationaboutthehousingorenclosure,tracelengths,keep-outareas;designatewhichcomponentsneedtobeontopoftheboard;includetuninginformation,componentvalueranges,thermalinformation,controlledimpedancelines,notes,briefcircuitoperatingdescriptions…(andthelistgoeson).TrustNoOneIfyou’renotdoingyourownlayout,besuretosetasideampletimetogothroughthedesignwiththelayoutperson.Anounceofpreventionatthispointisworthmorethanapoundofcure!Don’texpectthelayoutpersontobeabletoreadyourmind.Yourinputsandguidancearemostcriticalatthebeginningofthelayoutprocess.Themoreinformationyoucanprovide,andthemoreinvolvedyouarethroughoutthelayoutprocess,thebettertheboardwillturnout.Givethedesignerinterimcompletionpoints—atwhichyouwanttobenotifiedofthelayoutprogressforaquickreview.This“loopclosure”preventsalayoutfromgoingtoofarastrayandwillminimizereworkingtheboardlayout.Yourinstructionsforthedesignershouldinclude:abriefdescriptionofthecircuit’sfunctions;asketchoftheboardthatshowstheinputandoutputlocations;theboardstackup(i.e.,howthicktheboardwillbe,howmanylayers,detailsofsignallayersandplanes—power,,analog,digital,andRF);whichsignalsneedtobeoneachlayer;wherethecriticalcomponentsneedtobelocated;theexactlocationofbypassingcomponents;whichtracesarecritical;whichlinesneedtobecontrolled-impedancelines;whichlinesneedtohavematchedlengths;componentsizes;whichtracesneedtokeptawayfrom(ornear)eachother;whichcircuitsneedtobekeptawayfrom(ornear)eachother;whichcomponentsneedtobecloseto(orawayfrom)eachother;whichcomponentsgoonthetopandthebottomoftheboard.You’llnevergetacomplaintforgivingsomeonetoomuchinformation—toolittle,yes;toomuch,no.Alearningexperience:About10yearsagoIdesignedamultilayersurface-mountedboard—withcomponentsonbothsidesoftheboard.Theboardwasscrewedintoagold-platedaluminumhousingwithmanyscrews(becauseofastringentvibrationspec).Biasfeed-throughpinspokedupthroughtheboard.Thepinswerewire-bondedtothePCB.Itwasacomplicatedassembly.SomeofthecomponentsontheboardweretobeSAT(setattest).ButIhadn’tspecifiedwherethesecomponentsshouldbe.Canyouguesswheresomeofthemwereplaced?Right!Onthebottomoftheboard.Theproductionengineersandtechnicianswerenotveryhappywhentheyhadtoteartheassemblyapart,setthevalues,andthenreassembleeverything.Ididn’tmakethatmistakeagain.Location,Location,LocationAsinrealestate,locationiseverything.Whereacircuitisplacedonaboard,wheretheindividualcircuitcomponentsarelocated,andwhatothercircuitsareintheneighborhoodareallcritical.Typically,input-,output-,andpowerlocationsaredefined,butwhatgoesonbetweenthemis“upforgrabs.”Thisiswherepayingattentiontothelayoutdetailswillyieldsignificantreturns.Startwithcriticalcomponentplacement,intermsofbothindividualcircuitsandtheentireboard.Specifyingthecriticalcomponentlocationsandsignalroutingpathsfromthebeginninghelpsensurethatthedesignwillworkthewayit’sintendedto.Gettingitrightthefirsttimelowerscostandstress—andreducescycletime.Power-SupplyBypassingBypassingthepowersupplyattheamplifier’ssupplyterminalstominimizenoiseisacriticalaspectofthePCBdesignpro