AN-903A Comparison of Differential Termination Techniques

1 ApplicationReportSNLA034B August1993 RevisedApril2013AN- 903a August1993 RevisedApril2013AN- 903a ComparisonofDifferentialTerminationTechn iquesSubmitDocumentationFeedbackCopyrigh t 1993 2013, ,thesubjectoftransmissionlinetermination canbesomewhatconfusingsincetherearesoman ywaysin whicha , samplewaveformgeneratedbya setupconsistingofa functiongeneratorwhosesignalsaretransmit tedacrossa twistedpaircablebya : Unterminated Series/Backmatch Parallel AC Power(Failsafe) AlternateFailsafe Bi-DirectionalForthepurposesofdiscussion ,popularTIA/EIA-422driversandreceivers,s uchastheDS26LS31andDS26LS32A, , wherea DS26LS31differentialdriveranda DS26LS32 Adifferentialreceiverhavebeenconnected(u singa twistedpaircable)togetherwithouta ,theDS26LS31driver'sworstcaseloadis theDS26LS32 Areceiver' ,TIA/EIA-422-A(RS-422)standarddefinesthe DS26LS32A'sminimuminputresistancetobe4 k ,thedriver'sworstcaseload,asseenin Figure1, is then4 k.

2 Intheunterminatedconfiguration,theDS26LS 31driveris onlyrequiredtosourcea minimalamountofcurrentin ordertodrivea turnminimizesthedriver' ,the4 k driveroutputloadresultsin a higherdriveroutputswing(thanif thedriverwasloadedwith100 ) whichin noconfusion,noisemarginis ,if a receiverwasusedwhichcompliestoTIA/EIA-48 5(RS-485), ascomparedto4 k ComparisonofDifferentialTerminationTechn iquesSNLA034B August1993 RevisedApril2013 SubmitDocumentationFeedbackCopyright 1993 2013, terminationelementattheDS26LS32A'sinputs alsoensuresthatthereceiveroutputisin a knownlogicstatewhenthetransmissionlineis in theidleoropenlinestate(receiverdependent ).Thisconditionis , (VID) toa valuegreaterthan200mVwhenthelineis,forex ample,idle(un-driven).Thisbiasis significantin thatitrepresentstheminimumensuredVIDrequ iredtoswitchthereceiveroutputintoa :A completediscussionofreceiverfailsafecanb efoundin theAN-847 FAILSAFEB iasingofDifferentialBusesApplicationRepo rt(SNLA031).

3 Thereare,however, signalpropagatingdowna transmissionlinewillbereflectedbacktowar dsthesourceif theoutboundsignalencountersa mismatchin , themismatchoccursbetweenthecharacteristi cimpedanceofthetwistedpair(typically100 ) andthe4 k a turngeneratesadditionalreflectionsbackto wardthereceiver, a August1993 RevisedApril2013AN- 903a ComparisonofDifferentialTerminationTechn iquesSubmitDocumentationFeedbackCopyrigh t 1993 2013, Figure2. A steadystateconditionis a , ,therefore, ,a shortcablelengthensuresthatthetimerequir edforthereflectionstosettleis kepttoa generallycharacterizedtobeeithersignalin gratesbelow200kbits/secorwhenthecabledel ay(thetimerequiredforanelectricalsignalt otransversethecable)is substantiallyshorterthanthebitwidth(unit interval)orwhenthesignalrisetimeis morethanfourtimestheonewaypropagationdel ayofthecable(thatis,nota transmissionline).

4 Asa generalrule,if thesignalrisetimeis greaterthanfourtimesthepropagationdelayo fthecable,thecableis nolongerconsidereda ,signalterminationis theapplicationonlyrequireslowspeedoperat ionovershortdistances, ,RS, arechosensuchthattheirvalueplustheimpeda nceofthedriver' stillencounteredatthefarendofthecable(re ceiverinputs). ,whenthatsignalpropagatesbacktothedriver thereflectionis onlyonereflectionbeforethedrivensignalre achesa takesforthedrivensignaltoreachsteadystat eis stillminimizedduetothelightloadingpresen tedbythe4 k effectivelyloadedwiththereceiver' ,thedriveroutputimpedancescanvary,dueton ormalprocessvariations, problemwhichinvolvesreplacinglinedrivers ,thereis a chancethatthedesignermighthavetoreworkth eboardin ordertoensurethattheRSmatchesthenewdrive r' ,seriesterminationis a secondreceiver(multidropapplication)

5 Waslocatedhalfwaybetweenthedriverandrece iveratthefarendofthecable, problemwouldnotexistin a pointtopointapplicationwhereonlyonerecei veris ComparisonofDifferentialTerminationTechn iquesSNLA034B August1993 RevisedApril2013 SubmitDocumentationFeedbackCopyright 1993 2013, ,thereis ,thismismatchis causedbyasignalpropagatingdowna 100 cablesuddenlyencounteringa 4 k somewhatterminatedbythedriver' terminatedbecausethecombinedimpedanceoft heseriesresistor(RS) andthedriver' unterminatedsignalwaveform,thewaveformse enin Figure4 (tomaintaindataintegrity),it isimperativethattheroundtripcabledelaybe keptmuchlessthanthetimeunitinterval(TUI definedtobetheminimumbitwidthorthe distance betweensignaltransitions).Inotherwords,s eriesterminationshouldbelimitedtoapplica tionswherethecablelengthsareshort(tomini mizeroundtripcabledelays)andthedataratei s low(tomaximizetheTUI).

6 Andtoa lesserdegree,theseriesterminationoptionm aynotbetheidealchoicefroma costperspectivein thatit August1993 RevisedApril2013AN- 903a ComparisonofDifferentialTerminationTechn iquesSubmitDocumentationFeedbackCopyrigh t 1993 2013, ,parallelterminationemploysa resistoracrossthedifferentiallinesatthef ar(receiver) (ZO) generalrule,however,it is usuallybettertoselectRTsuchthatit is slightlygreaterthanZO. typicallychosentobeequaltoZO. Whenover-terminationisusedRTis typicallychosentobeupto10%largerthanZO. mind,however,thatthereis , ' ,parallelterminationcaneffectivelyremove allreflections; ComparisonofDifferentialTerminationTechn iquesSNLA034B August1993 RevisedApril2013 SubmitDocumentationFeedbackCopyright 1993 2013, Figure6 eitherhighspeed(10Mb/s),orlongcablelengt h(upto4000feet), definedasa distributionsystemcomposedofonedriverand uptotenreceiversspreadoutalongthecableas definedin (oppositethedriver)ofthecableandeffectiv elyterminatesthesignalatthatlocation, ' thatstublengths,asmeasuredby I in Figure5, , ,it doesplacea ofthedriversrisetimein lengthtominimizetransmissionlineeffects, 100 resistortobeusedwhenthedifferentiallinei s ,applicationswhichusea TIA/EIA-422-AdriversuchastheDS26LS31orDS 26C31arecommonlyterminatedwith100 parallelterminationeliminatesallreflecti ons, a a (4k ) (typically100 )

7 Broughtonbyparallelterminationreducesthe driver' ,evenwiththisreduction,thereis ,openinputreceiverfailsafeoperationwasen suredbecauseofinternalcircuitry(receiver dependent)whichbiasesthedifferentialinpu tvoltage(VID) toa (VID), is greaterthan+200mV,theoutputoftheDS26LS32 Areceiverremainsin a ,parallelterminationcannotsupportopeninp utreceiverfailsafewhenthetransmissionlin eis in discussedin muchgreaterdetaillaterin thesectionwhichdescribespowerandalternat efailsafetermination(seeAN-847[SNLA031] formoreofinformationonfailsafebiasingdif ferentialbuses).5 ACTerminationTheeffectivenessofparallelt erminationis ,RT(seeFigure5), isoftentoolargein ,paralleltermination'sis notabletoensurereceiverfailsafeduringidl ebusstateswhichin ,however, representationofanACterminateddifferenti allineis shownin August1993 RevisedApril2013AN- 903a ComparisonofDifferentialTerminationTechn iquesSubmitDocumentationFeedbackCopyrigh t 1993 2013, 150 (cableZOdependent)andis selectedtomatchthecharacteristicimpedanc e(ZO) , ontheotherhand,is selectedtobeequaltotheroundtripdelayofth ecabledividedbythecable' (Cableroundtripdelay)/ ZO(1)Forthisexample:CableLength= 100feetVelocity= 100 Therefore,CT (100ft 2 )/100 or 3,400pFFurther,theresultingRCtimeconstan tshouldbelessthanorequalto10%oftheunitin terval(TUI).

8 Parallelterminationduringtransitions, reducedattheexpenseofa Figure8 is 300kHzwhereasit ComparisonofDifferentialTerminationTechn iquesSNLA034B August1993 RevisedApril2013 SubmitDocumentationFeedbackCopyright 1993 2013, Figure8 ,thecapacitorCTbehavesasa shortcircuitandconsequently,theloadprese ntedtothedriveris essentiallyRT. However,oncethedrivensignalreachesitsint endedlevels,eithera logicHIGH orlogicLOW, duetothefactthatthedriveris nowloadedwitha largereceiverinputresistancetypicallygre aterthan4 k ; versusthetypicalRTof100 120 . Thisreducedloadingconditionincreasesthes ignalswingofthedriverandresultsin ,thereceiver'sinternalpullupandpulldownr esistorswillbiastheoutputintoa ,besidesminimizingDCloopcurrent,preventi nglinereflections,andincreasingnoisemarg in, ,therearedisadvantagesin theresultoftheimpactthatRTandCT, together,haveuponthedrivensignal' limitedis dependentupontheselectionofRTandCT.

9 LongRCtimeconstantswillhavea greaterimpactuponthedrivensignal'smaximu mdatarate, ,thetransmissionlinesbestsuitedforACterm inationaretypicallylowspeedcontrollinesw herelevelsensitivityis ,thepartcountrequiredbyACterminationcanp utit ata disadvantagein intendedprimarilytoeliminatethelargeDClo opcurrentinherentin ,ontheotherhand,addressesparalleltermina tion' foranillustrationofa , whenthebusis idle,almostassuresthatthereceiveroutputw illnotbein a duetotheinsufficientvoltageacrossRT(onth eorderof1 mV 5 mV)ascausedbythereceiver' ,regardlessofwhetherthebusis openoridle,a minimumof+200mV(withrespecttotheinverted receiverinput)mustbedevelopedacrossRT. Thesolepurpose,then,ofRAandRBis toestablisha voltagedividerwherebyatleast+200mVwillbe droppedacrossRT. A completeexplanationofselectioncriteriafo rresistorvalues(RAandRB) canbefoundin AN-847(SNLA031).

10 TheadditionofexternaIreceiverfailsafebia singresistors,however, Theincreaseddriverloadingdecreasesthedri ver'soutputswingand,in turn, thattheextracostandsubsequenthandlingoft woadditionalresistors(excludingRT) mightoutweighpowertermination'sadvantage sin August1993 RevisedApril2013AN- 903a ComparisonofDifferentialTerminationTechn iquesSubmitDocumentationFeedbackCopyrigh t 1993 2013, 'sabilitytooperatein carefulselectionofRCandRDwilldetermineho wmuchmorecommonmodevoltagea magnitude,howevermostintegratedcircuitss upportabsolutemaximumratingthatexceedthe the35V 45 Vrangeonthecable,whilestillhonoringthe25 Vlimitin , whileRAandRBare47k . betweenthereceiverinputpins,andalsoallow sthepullupandpulldownresistorstobeincrea sedin valueto47k . Thiscapabilitylendsitselfwelltoapplicati ons,suchasfactorycontrolandbuildingtobui ldingdatatransmission,wherethecommonmode rangecanoccasionallyexceed ,idle, ,thefailsafeoptionis Inthisshortcondition, ,therefore, ,withtheadditionofRCandRD,alsoresultsin 'sabilitytowithstandlargercommonmodevolt ages,it mightalsonegativelyimpactthereceiver' ,forexample,a (VID) requiredtoswitchthereceiveris normally|200mV|.