Considerations for PCB Layout and Impedance …

1 ApplicationReportSLLA311 (TOSAs)andlaserdriversmayhavedifferentre sistancesina givenapplication,sothereflectioncouldbew orseif (PCB) mismatchedtraceandnon-terminatedTOSA, February2011 ConsiderationsforPCBL ayoutandImpedanceMatchingDesignin OpticalModulesSubmitDocumentationFeedbac k 2011,TexasInstrumentsIncorporatedROSATOS AMCUPost Amp (PA)Laser , ,impedancematchingis ,opticalmodulesareanapplicationwithsever alconstrainingfactors:frequencyoverGbps; variationsin thelaserdrivermodel;theactualtransmissio nlines;and,mostimportantly, result,evenwithgoodmodelstopredicttheope ratingconditionswithrelativeaccuracy, thisdocument,impedancemismatchresultsin 10-Gdirectmodulaterlaser(DML) (TOSA)in (ROSA)elementsAtpoint2,thereflectionis ,thereflectionsarea ,a portionofthetransmittedsignalisreflected backtowardsthesignalsource,andit canpropagatebackandforthin thetraceuntilit , thereflectedenergyis toohigh, OpticalSLLA311 February2011 ModulesSubmitDocumentationFeedback 2011,TexasInstrumentsIncorporatedGL=VVRE FLIN=ZZZ + ZL0L0-RM17W22 is definedasshownin Equation1:(1)Inthisequation,Z0is thetransmissionlineimpedance.

2 Thisfactoris usuallya constantwithanindustrynormalizedvaluesuc has50 or100 . Ina matchedsystem,whentheloadimpedanceZLmatc hesthetransmissionlineimpedance, L= 0 ,theinputandoutputofthelimitingamplifier andtheoutputofthetransimpedanceamplifier areall100- differentialimpedance;thus,thetransmissi onlinesare100- differential,andmatchingis ,thetransmitterpartis distributedfeedback(DFB)orFabry-Perot(FP )laseris a low-resistancecomponent,typicallyranging from7 to10 . TheflexiblePCBcableiscommonlya 25- , a reflectionattheinterconnectionbetweenthe flexcableandtheTOSA atpoint4;refertoFigure1. Forreducingsuchreflection,sometypesofTOS A sintegrateresistorstoincreasetheTOSA Impedance ,asillustratedin closeto25 andthetransmissionlinecanbemoreeasilymat chedtotheTOSA witha ,manyopticaldesignersprefertousedifferen tialmoderatherthansingle-endedmodetoavoi dissueswithcommon-modenoiseandelectromag neticinterference(EMI).

3 3 SLLA311 February2011 ConsiderationsforPCBL ayoutandImpedanceMatchingDesignin OpticalModulesSubmitDocumentationFeedbac k 2011,TexasInstrumentsIncorporatedA) Single-Ended ModeB) Differential FmLaserBIASPDVCCVCCMOD+MOD+MOD-MOD-VCCVC CIMOD-IMOD+ comparesanopticalmoduledriverin bothsingle-ended(a) anddifferential(b) differentialtransmissionlinesin differentialmode,a fully-matchedTOSA withtwo20- internalresistors,togetherwiththelaserre sistance,resultsin a loadresistancecloseto50 .Onepotentialproblemis thattheresistorsconsumecurrentandheatthe laser, (ER)andaverageopticalpower(PAV) ,especiallyforSFP+modulesthatrequirea totalpowerdissipationoflessthan1 , , OpticalSLLA311 February2011 ModulesSubmitDocumentationFeedback 2011,TexasInstrumentsIncorporatedGNDDIN+ DIN-GNDFLTVCCMOD+MOD-VCCBIASSDASCKTSRZTC DISBGVMONBMONPCOMPPDONET8501VC8C9100- Diff TLW100- Diff TLWL1L2L3L4 MonitorPhotodiodeVCSELVCCL2L3L4L525W25W5 0W50W50W35W35W220W220W50 WZOUTSMA_4 SMA_2 ZSOURCEZLOADI mpedance Transfer ,suchasthatillustratedin Figure4, ,theoutputimpedanceis 100- differential.

4 Thetransmissionlineis also100 , andtheloadis 100- illustratestheimpedancetransferconfigura tionfora February2011 ConsiderationsforPCBL ayoutandImpedanceMatchingDesignin OpticalModulesSubmitDocumentationFeedbac k 2011,TexasInstrumentsIncorporatedZ= (220 //OUT12Z+ 50) // 35 LOADMOD+MOD+MOD-MOD-50- Diff FmLD12 ZOUTTOSA: ~ 50- differentialoutputandthetestloadimpedanc eis 100- differentialin ordertotransfertheimpedancebetweenthesou rceandload,usingthe and220- resistorsanda 50- seriesresistor, is givenin Equation2:(2)ZLOADis 100- differential;therefore,wecalculateZOUT byusinghalfofZLOAD. ThefinalZOUT valueis 27 ,whichis showstheONET1101 LdrivinganunmatchedTOSAin optimizedtodrivea 50- low-powerdesign,it hasa 500- Impedance (approximately),thereflectionis muchworseif thereis todrivetheloadimpedancecloseto50 , wecanderiveEquation3:ZOUT= R1//(R3+ R4+ R2// RTOSA)(3)Withtheresistorvaluesshown,ZOUT is approximately35.

5 6 ConsiderationsforPCBL ayoutandImpedanceMatchingDesignin OpticalSLLA311 February2011 ModulesSubmitDocumentationFeedback 2011,TexasInstrumentsIncorporatedEye diagram improved; MM = 25%, I= 121 mA PRBS-31, signal GbpsCCR1 = 120 WDrive directlyBad eye diagram; MM = 0%, I= 109 mA PRBS-31, signal GbpsCC50- Diff TLW50- Diff FmVCCMOD-MOD+MOD+MOD-VCCM onitorPhotodiodeBLM15HG102SN1 BLM15HG102SN1 BLM15HG601SN1 BLM15HG601SN1 BLM15HG102SN1 BLM15HG102SN1 BLM15HG601SN1 BLM15HG601SN1120W50- Diff TLW50- Diff FmVCCMOD-MOD+MOD+ newbacktermination:ZS ,highlightedin theblueoutlinedarea(1)in Figure7. ThenewZS willbe90 . Forbestresults, newZL , shownin theblueoutlinedarea(2)inFigure7.

6 ThenewZL willbeverynear9 . Withthetwoserial20- resistors,theimpedanceisapproximately50 , , , andFigure9 showvarioustestresultswiththeONET1101 LandNX8341 TBTOSA(2 10- internalresistors). :PoorPerformance7 SLLA311 February2011 ConsiderationsforPCBL ayoutandImpedanceMatchingDesignin OpticalModulesSubmitDocumentationFeedbac k 2011,TexasInstrumentsIncorporatedTwo series 10- resistorsWEye diagram improved, MM = 25%, I= 131 mA, PRBS-31, signal GbpsCCEye diagram improved, MM = 25%, I= 140 mA, PRBS-31, signal GbpsCCR2 = 180W120W10W10W50- Diff FmVCCMOD-MOD+MOD+MOD-VCCM onitorPhotodiodeBLM15HG102SN1 BLM15HG102SN1 BLM15HG601SN1 BLM15HG601SN1 BLM15HG102SN1 BLM15HG102SN1 BLM15HG601SN1 BLM15HG601SN1120W10W10W50-Diff FmVCCMOD-MOD+MOD+.

7 ImprovedPerformanceFromthesetests,wecans eethattheimpedancetransfercircuitworksqu itewelltoimprovethemodulereflectionperfo rmance, , OpticalSLLA311 February2011 ModulesSubmitDocumentationFeedback 2011,TexasInstrumentsIncorporated| Loss f, L| = e-(f +f) LaaSD Where:f: Frequency: Skin effect loss factor: Dielectric loss factorL: Channel lengthaaSD051015202530354045505560------ ------Differential Stripline Attenuation (dB)0123456 Frequency (GHz)12-in Stripline24-in Stripline36-in Stripline48-in , , (MicrostripvsStripline) February2011 ConsiderationsforPCBL ayoutandImpedanceMatchingDesignin OpticalModulesSubmitDocumentationFeedbac k 2011,TexasInstrumentsIncorporatedZ( ) = 2 Z1 ((shZ( ) = 2 Z1 ((shZ ( ) =0W In(( W + t + ( ) =0W In(( W + t 60erW (mils) = h t e-+ (mils)))))))))

8 = h t e-erZ0 60 StubDiscontinuities+ signal-signalLogic statuschange+ signal-signalNoise pulse duringlogic status changesFormerswitchingpoint(a) Differential Pairs: Same Length(b) Differential Pairs: Different differentialsignalpair;notethatsoftwaret oolsaremoreaccurate, (4)(5)(6)(7)(8)(9) Avoidstubsanddiscontinuities(similartoth oseillustratedin Figure12) in (a)anddifferentlengths(b). If thedifferentialsignalpairsarenotthesamel ength, ,asshownin Figure13(b), a OpticalSLLA311 February2011 ModulesSubmitDocumentationFeedback 2011, Useradialbends,shownin Figure14, if radialbendis thebestoptionforPCBlayoutin > 5 Wis ,somereflectionis ,thesedocumentsareavailablefordownloadfr omtheTIwebsite( ).

9 ONET8501 PBproductdatasheet,TexasInstrumentsliter aturenumberSLLS910. ONET8501 Vproductdatasheet,TexasInstrumentslitera turenumberSLLS837B. ONET1101 Lproductdatasheet,TexasInstrumentslitera turenumberSLLS883. OKI10-GDFBOL3356l_z25productdatasheet. NECNX8346TB,NX8341,ADN2526,ADN2525series productdatasheet. Weiler, ,A.(1996).ApplicationandDesignConsiderat ionsfortheCDC5 XXPlatformofPhase-LockLoopClockDrivers. TexasInstrumentsapplicationreport,litera turenumberSCAA028. PCBL ayoutGuidelinesforDesigningwithAvagoSFP+ Transceivers. ConsiderationsforHigh-SpeedPCBT rackDesignin 10-Gb/sSerialDataTransmission. High-speedPCBdesignconsiderations. February2011 ConsiderationsforPCBL ayoutandImpedanceMatchingDesignin OpticalModulesSubmitDocumentationFeedbac k 2011,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedanditssubsidi aries(TI)

10 Reservetherighttomakecorrections,modific ations,enhancements,improvements, s accordancewithTI s ,testingofallparametersofeachproductis , ,eitherexpressorimplied,is grantedunderanyTIpatentright,copyright,m askworkright,orotherTIintellectualproper tyrightrelatingtoanycombination,machine, orprocessin licensefroma thirdpartyunderthepatentsorotherintellec tualpropertyofthethirdparty,ora TIdatabooksordatasheetsis permissibleonlyif reproductionis withoutalterationandis accompaniedbyallassociatedwarranties,con ditions,limitations, safety-criticalapplications(suchaslifesu pport)wherea failureoftheTIproductwouldreasonablybeex pectedtocauseseverepersonalinjuryordeath , thesafetyandregulatoryramificationsofthe irapplications,andacknowledgeandagreetha ttheyaresolelyresponsibleforalllegal,reg ulatoryandsafety-relatedrequirementsconc erningtheirproductsandanyuseofTIproducts in suchsafety-criticalapplications, , military/aerospaceapplicationsorenvironm entsunlesstheTIproductsarespecificallyde signatedbyTIasmilitary-gradeor"enhancedp lastic.