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AN-1429APPLICATIONNOTEOneTechnologyWayP.
O.
Box9106Norwood,MA02062-9106,U.
S.
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Tel:781.
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461.
3113www.
analog.
comDesignConsiderationsandSolutionsforHeadphoneDriversinMobilePhonesByDavidGuoRev.
0|Page1of8INTRODUCTIONHighfidelitydescribesthequalityofaudioequipment.
Highfidelityaudioequipmenthasidealtotalharmonicdistortionplusnoise(THD+N)performanceandaccuratefrequencyresponse,resultinginexcellentsubjectivetestresults.
Portablehighfidelityaudioequipmentbringscustomersahigherqualitymusiclisteningexperience.
Comparedtomobilephones,however,portablehighfidelityaudioequipmentisinconvenientlylarge.
Thoughgivenincreasedmarketdemandandtechnicaladvances,itisnowpossibletointegratethehighfidelityfunctionintothinnermobilephones.
Typically,audiodigital-to-analogconverters(DACs)cannotdrivelowimpedanceheadphoneswell.
However,toreachqualityperformance,operationalamplifiersareusedwithaudioDACsforsignalconditioning,includingcurrenttovoltage(ItoV)conversion,filter,attenuation,anddifferentialtosingle-endedconversion.
Theoperationalamplifiersmusthavelownoise,lowdistortion,andstrongdrivecapability.
Theoperationalamplifiermustalsoperformwellinsubjectivetestingwithcustomers.
ManyAnalogDevices,Inc.
,operationalamplifiersarereputablewithmusicfans,liketheAD797,OP275,AD8620,andADA4627-1.
Inportablemobilephoneapplications,thequiescentcurrentandpackageofadeviceisimportant.
TheADA4841-2,ADA4896-2,ADA4075-2,ADA4807-2,andAD8397haveidealnoiseanddistortionperformances.
Thisapplicationnotefocusesonthecircuitdiscussionandoperationalamplifierrecommend-dationsforthishighfidelityheadphonedriverapplication.
SIGNALCHAINInthistypeofapplication,usehighperformanceandlowpoweraudioDACstodeliveradynamicrange(DNR)ofupto127dBandTHD+Nof120dB.
SomehighperformanceaudioDACscanbeconfiguredasavoltageoutputorcurrentoutput.
ConfigurecurrentoutputforbetterDNRandTHD+N.
Foravoltageoutputconfiguration,theconditioningcircuitisadifferentamplifiercircuit,whichconvertsthedifferentialsignalfromtheRchannelorLchanneltoasingle-endedsignal(seeFigure1).
Figure1.
VoltageOutputConfigurationForacurrentoutputconfiguration,implementanItoVcircuittoconvertthedifferentialcurrentsignalfromtheRchannelandtheLchanneltothedifferentialvoltagesignal,followedbyadifferenceamplifiercircuit(seeFigure2).
Inmobilephoneapplications,thepowerconsumptionofdevicesiskey.
Inthisapplicationnote,a±5Vpowerischosenastheexampleforanalysisconvenience.
Figure2.
CurrentOutputConfigurationRDACV+V–DIFFERENTIALTOSINGLE-ENDVOLTAGEOUTPUTLV+V–15032-001RDACI+I–DIFFERENTIALTOSINGLE-ENDITOVCURRENTOUTPUTLI+I–15032-002AN-1429ApplicationNoteRev.
0|Page2of8TABLEOFCONTENTSIntroduction1SignalChain1RevisionHistory2ItoVStage.
3OperationalAmplifierRecommendationsforItoVStageinMobilePhones3THD+Nvs.
VOUTMeasurement4THD+Nvs.
VOUTResult.
4HeadphoneBasics4OutputStage(First-OrderLow-PassFilter)6OutputStage(Second-OrderLow-PassFilter)6OperationalAmplifierRecommendationsforOutputStage.
.
6THD+Nvs.
VOUT/IOUTMeasurement.
7THD+Nvs.
VOUT/IOUTResult.
7Summary8REVISIONHISTORY10/2016—Revision0:InitialVersionApplicationNoteAN-1429Rev.
0|Page3of8ITOVSTAGEAnidealItoVconverterforacurrentoutputDACisaresistortoground.
However,mostDACsdonotoperatelinearlywithvoltageattheoutput.
ItisstandardpracticetooperateanoperationalamplifierasanItoVconverter,creatingavirtualgroundattheinvertinginput.
Normally,theoperationalamplifieroutputstageabsorbsclockenergyandcurrentsteps.
However,Figure3showstheCFcapacitorshuntshighfrequencyenergytogroundwhilecorrectlyreproducingthedesiredoutputwithextremelylowTHDandintermodulationdistortion(IMD).
Figure3.
ItoVStageTherearefourcomponentsinthecircuit:RFisthefeedbackresistor.
Forlowestnoise,maximizegaininthefirststage(ItoV).
However,distortionisrelatedtotheopen-loopgain(AOL)oftheoperationalamplifier.
ThehigherAOL,thebetterthedistortionperformance.
Normally,AOLisspecifiedwithincertainoutputvoltages(seeFigure3).
TheADA4896-2AOLmustbe100dBminimumwhentheoutputis4Vto4Vat±5Vpowersupply,orwhendistortionincreasesbeyond4Vto4V.
AssumingthemaximumoutputcurrentofaDACis1mA,then,kΩ4mA1V4===OUTOUTFIVRCFisthefeedbackcapacitorinparallelwithRF.
CFandRFformapoleinthetransferfunction,thereforethecutofffrequency(fC)ofthelow-passfilterisFFCCRfπ=21AssumingfCis100kHz,then,pF390kΩ100kΩ42121≈**π=π=CFFfRCRSistheresistorinserieswithCF.
Typically,RS=100forbetterstabilityandTHD+Nperformance.
VBIASisthevoltagebias.
Typically,audioDACsgenerateadcoffsetcurrent.
Formaximizingtheeffectiveoutputsignal,addaVBIASatthenoninvertinginputterminaltocancelthedcvoltagebytheDACdcoffsetcurrent.
OPERATIONALAMPLIFIERRECOMMENDATIONSFORITOVSTAGEINMOBILEPHONESThekeyspecificationsofoperationalamplifiersastheItoVstageinheadphonedriversincludepowersupply,IQ,voltageandcurrentnoise,THD+N,package,common-moderejectionratio(CMRR),powersupplyrejectionratio(PSRR),AOL,andslewrate.
TheADA4841-2,ADA4896-2,ADA4075-2,andADA4807-2arerecommendedforItoVconversion.
Keypointsaboutthesedevicesincludethefollowing:Alldevicesarelownoise,lowpower,andhaveasmallpackage.
ADA4807-2usesthelowestIQtoachievelownoiseandrail-to-railinput/output(RRIO)anditintegratesthedisablefunction,whichcanfurtherdecreasethepowerconsumption.
TheADA4075-2specifiedpowersupplyis9Vminimum,isnotarail-to-railoutput(RRO),andhasthesmallestpackage.
THD+Nisthekeyspecification.
SeeFigure4andFigure5forthetestcircuitandtestresultofTHD+N,respectively.
Figure4.
THD+NTestWithoutLoadI+ITOVVBIASRfRSCf15032-0031kΩ1kΩSYS-27121kHzBNC-UNBALANCEBNC-UNBALANCE+5V–5V15032-004AN-1429ApplicationNoteRev.
0|Page4of8THD+NVS.
VOUTMEASUREMENTTodeterminethemaximumoutputwithoutTHD+Ndegrading,measureTHD+Nvs.
VOUT.
Typically,anaudioDACcurrentoutputactsasaresistorinserieswithavoltagesource.
Thatis,theItoVcircuitcanbeconsideredaninvertingamplifiercircuit.
Forsimplicity,gain=1todeterminetheTHD+Nperformanceofoperationalamplifiers(seeFigure5).
ThefollowingdetailstheproceduretomeasureTHD+Nvs.
VOUTusingtheSYS-2712fromAudioPrecision:Thepowersupplyis±5V.
TheSYS-2712analoganalyzergeneratesa1kHzsinewaveastheinputoftheamplifiercircuit.
TheoutputoftheamplifiercircuitfeedsintotheSYS-2712toobtainTHD+Ndata.
ThebandwidthoftheSYS-2712analyzerisconfiguredas22kHz.
TofindTHD+NattheoutputrangeintheSYS-2712evaluatingsoftware,theinputoftheanaloganalyzerisconfiguredasautorange,thatis,theinputstagegainoftheanalyzerincreasesautomaticallybythedifferentinputsignals,including40mV,160mV,300mV,600mV,1.
2V,2.
5V,and5V.
Typically,thelargerthegain,theworsethenoiseleveloftheanalyzerandTHD+N.
THD+NVS.
VOUTRESULTFigure5showsTHD+Nvs.
VOUTresultsfortherecommendedoperationalamplifiers.
WhenVOUTislessthan1.
2Vrms,THD+Nperformanceofthefouroperationalamplifiersissimilar.
WhenVOUTislargerthan1.
2Vrms,theanaloganalyzeroftheSYS-2712isswitchedto2.
5Vscale,increasingthegainofinternalprogrammablegainamplifier(PGA).
ThenoiseworsensandTHD+Ndegradesslightly.
TheSYS-2712causesthisdeg-radation.
ThisisnotthetrueperformanceoftheoperationalamplifierbecausetheSYS-2712hasworsenoisewithalargerPGAgain.
AsVOUTcontinuestoincrease,THD+Ndegradesdramaticallyatthemaximumoutputvoltageoftheamplifier.
FortheADA4841-2,ADA4896-2,andADA4807-2,thevoltageisabout3.
5Vrms(4.
9Vpeak).
ThismatchestheRROfeatureofthesedevices.
FortheADA4075-2,thevoltageisabout2.
13Vrms(3.
0Vpeak).
Thismatchesthe2VoutputvoltagetotherailintheADA4075-2datasheet.
Figure5.
THD+NResultofRecommendedOperationalAmplifiersHEADPHONEBASICSThecharacteristicsofload(headphones)determinestheoutputstage.
Theheadphoneshavetwokeyspecifications:impedanceandsensitivity.
Impedanceistypicallymeasuredat1kHz.
Lowimpedanceheadphonesareintherangeof8Ωto32Ω.
Highimpedanceheadphonesareintherangeofabout100Ωto600Ω.
Astheimpedanceincreases,morevoltage(atagivencurrent)isrequiredtodriveit.
Asaresult,theloudnessoftheheadphonesforagivenvoltagedecreases.
Inrecentyears,impedanceofnewerheadphoneshasgenerallydecreasedtoaccommodatelowervoltagesavailableonbattery-poweredportableelectronics,likemobilephones.
Lowerimpedancemeansheavierloadtotheoperationalamplifier,whichrequiresoperationalamplifierstohavelargercurrentdrivecapabilitieswithoutdistortion.
Sensitivitymeasureshowloudtheheadphonesareforagivenelectricaldrivelevel.
Itcanbemeasuredindecibelsofsoundpressurelevel(SPL)permilliwatt(dB/mW)ordecibelsofSPLpervolt(dB/V).
Byanalyzingthewavefileofaliverecording,themaximumSPLcanreach120dB.
TheaverageSPLislessthan100dB.
Findtherequiredpeakpowerusingthefollowingformula:=1010ySensitivitSPLRequiredP–120–100–80–60–40–2000.
114THD+N(%)OUTPUTVOLTAGE(V)15032-005ADA4841-2ADA4896-2ADA4075-2ADA4807-2ApplicationNoteAN-1429Rev.
0|Page5of8Table1detailsthekeyspecificationsofsomeheadphones.
Theimpedancerangesfrom8Ωto600Ω.
Therequiredpeakpower,peakvoltage,andpeakcurrentarelisted.
Therequiredaveragepowerislessthan2mW.
Toreproducetheeffectofliverecording,theheadphonedrivermustoutputmorepower.
SeeTable1.
Forlowimpedanceheadphones,therequiredpeakcurrentcanreach80mAmaximum,andTHD+Nofthecurrentcannotdegrade.
Forhighimpedanceheadphones,theoutputvoltagemustbehigh.
Forexample,usingtheDT880fromBeyerDynamic(600Ω)requiresanoperationalamplifieroutputof12V,whichisimpossibleina±5Vsystem.
Iftheproducttargetsdrivinghighimpedanceheadphones,increasethepowersupplyoftheamplifiercircuit.
Table1.
Sensitivity,Impedance,PeakVoltage,andPeakCurrentofHeadphonesManufacturerModelSensitivity(dB/mW)Impedance(Ω)Frequency(Hz)AveragePower(mW)PeakPower(mW)PeakVoltage(V)PeakCurrent(mA)SONYXBA-410883to280000.
15815.
8490.
35644.
510Audio-TechnicaATH-CHX71001615to220001.
000100.
0001.
26579.
057ShureSE2151072022to175000.
20019.
9530.
63231.
585AppleEarpod109235to210000.
12612.
5890.
53823.
396GradoAliceM11003220to220001.
000100.
0001.
78955.
902CreativeAURVANAAir1023220to200000.
63163.
0961.
42144.
404KOSSPP1016010to250000.
79479.
4332.
18336.
385SennheiserHD6509830010to395001.
585158.
4896.
89522.
985BeyerDynamicDT880966005to350002.
512251.
18912.
27720.
461AN-1429ApplicationNoteRev.
0|Page6of8OUTPUTSTAGE(FIRST-ORDERLOW-PASSFILTER)Theoutputstageconvertsthedifferentialvoltagesignaltothesingle-endedvoltagesignal.
Figure6depictsacommondifferenceamplifiercircuit,alsoknownasasubtractorcircuit.
Figure6.
DifferentialSingle-EndedCircuitRGandRFdeterminethegainofthecircuit,whichisnormallygain<1.
Theresistanceofresistorsmustbesmall,typically1k,toavoidinducingmorenoise.
RFandCFformsapoleinthetransferfunction.
Therefore,fCofthelow-passfilterisFFCCRfπ=21RSdeterminestheoutputimpedanceorthedampingfactoroftheheadphonedriver.
Thehighdampingfactor(RL/RS)improvesthecontrolthesource(headphonedriver)hasovertheload(headphone).
Theimpedanceoftheheadphonedoesnothavepureresistanceandvariesbyfrequency.
AhigherRScaninducemoredistortion(especiallylowfrequencydistortion)becauseofthefrequencyvariedimpedance.
Fromaperformanceperspective,makeRSlow.
Generally,thedampingfactorremainsaboveone.
Fromasafetyperspective,higherRScanattenuatepowertoprotecttheheadphonefromdamaging,whilealsoprotectingtheamplifierincasetheoutputshortstoground.
Thiscanhappenwhenhotpluggingtheheadphones.
Figure7showstheADA4807-2(LFCSP)maximumpowerdissipationvs.
ambienttemperaturefora4-layerevaluationboard.
Figure7.
ADA4807-2MaximumPowerDissipationvs.
AmbientTemperaturefora4-LayerEvaluationBoardPowerdissipation(PD)canbe1.
274Wat85°Cambienttemperature.
Assuming±5Visthepowersupply,themaximumoutputvoltageis2Vrms.
RS=10,andRSisshortedtoground.
PDperchanneloftheADA4807-2is()LOUTLOUTSSSDRVRVVIVP2*+*=2PD=610mWConsideringtheADA4807-2isadual-channeldevice,thetotalPDis1.
22W.
Theoutputcurrentinthisexampleis200mA.
TheADA4807-2short-circuitcurrentis80mA,soRScanbesmallertoincreasethedampingfactor.
Foroperationalamplifierswithashort-circuitcurrentlargerthan200mA,RSmustbe10minimumtoavoidoperationalamplifierdamageinoutputshorttogroundstatus.
OUTPUTSTAGE(SECOND-ORDERLOW-PASSFILTER)Comparedtothefirst-orderlow-passfilter,thesecond-orderlow-passfilterhasasteeperroll-offresponse,removingmorenoiseoutofthespecifiedband.
Thetwo-polelow-passfilterwithdifferentialinputiseasilydesignedusingthedesignequationsforthesingle-endedinputmultiple-feedbacklow-passfilter.
ShowninFigure8,duplicatingthecomponentresultsinanequivalentfrequencyresponse.
Normally,thefiltergivesaBesselresponse,whichhasalinearphase.
Figure8.
Second-OrderMultipleFeedback(MFB)FilterOPERATIONALAMPLIFIERRECOMMENDATIONSFOROUTPUTSTAGEThekeyspecificationsfortheoutputstageoperationalamplifierissimilartotheItoVstage.
Theoutputstageoperationalamplifiermusttypicallyhave<100dBTHD+Nat32Ωload.
TheADA4841-2,ADA4807-2,andAD8397operationalamplifiersarerecommendedfortheoutputstageofheadphonedriversinmobilephones.
Therearefourpointstoconsider:Alloftheoperationalamplifiersarelownoise,lowpower,andsmallpackagedevices.
ADA4807-2achieveslownoiseandRRIObylowestIQ.
Italsointegratesadisablefunction,whichcanfurtherdecreasepowerconsumption.
RGRSRLRFCF15032-00600.
51.
01.
52.
02.
53.
03.
54.
0–40–25–105203550658095110125MAXIMUMPOWERDISSIPATION(W)AMBIENTTEMPERATURE(°C)15032-007C5SINGLE-ENDEDINPUTSINGLE-ENDEDOUTPUTDIFFERENTIALINPUTSINGLE-ENDEDOUTPUTR3R1R4C2C5R3R1R3R1R4C5C2R4C215032-008ApplicationNoteAN-1429Rev.
0|Page7of8AD8397typicallyhas<100dBTHD+Nat32Ωload.
Italsotestssubjectivelywell.
ThedisadvantageistheAD8397IQishigher,atabout12mA.
<100dBTHD+Nat32Ωloadisakeyspecification.
SeeFigure9andFigure10forthetestcircuitandtestresult,respectively.
SeetheADA4841-2,ADA4807-2,andAD8397datasheetsforrelevantspecifications.
THD+NVS.
VOUT/IOUTMEASUREMENTFigure9showsthecircuitusedtofindTHD+Nataheavyload.
ThefollowingdetailstheTHD+Nvs.
VOUT/IOUTmeasurement:Thepowersupplyis±3.
3V.
Thegainofthesubtractorisabout0.
243.
TheSYS-2712generatesa1kHzsinewaveastheinputoftheamplifiercircuit.
FeedtheoutputoftheamplifiercircuitintotheSYS-2712analoganalyzertodetermineTHD+N.
ConfigurethebandwidthoftheSYS-2712analyzerto22kHz.
TomeasureTHD+Nvs.
VOUT/IOUT,SYS-2712isconfiguredtooutputa100mVrmsto4Vrmssignal.
Figure9.
THD+NTestwith16ΩLoadTHD+NVS.
VOUT/IOUTRESULTFigure10andFigure11depictsTHD+Nvs.
VOUT/IOUToftherecommendedoperationalamplifiers.
Figure10.
THD+Nvs.
VOUToftheADA4841-2,ADA4807-2,andAD8397Figure11.
THD+Nvs.
IOUToftheADA4841-2,ADA4807-2,andAD8397Table2detailsthemaximumcurrentandmaximumoutputpowerwithparameterssetat<100dBTHD+Nat16load.
Figure11showsthefollowingresults:THD+NdegradesdramaticallywhenIOUTreachesacertaincurrentthreshold.
TheAD8397hashighcurrentdrivecapabilitiesat16load.
TheADA4807-2maximumoutputcurrentis42mAwithonly1.
2mAIQ,whichisrecommendedwhenprioritizingthepowerconsumptionbudget.
Inmostcases,2mWoutputpowerisenoughtodrivetheheadphones.
THD+N,inthiscase,isimportant;asdetailedinTable2,THD+Nat2mWofallrecommendedoperationalamplifiersis<100dB.
Table2.
MaximumCurrentandOutputPowerat16LoadParameterADA4841-2ADA4807-2AD8397MaximumCurrentat<100dBTHD+N29.
1mA42.
1mA125mAOutputPowerat<10dBTHD+N13.
6mW28.
4mW250mWTHD+Nat2mWOutputPower102.
7dB102.
7dB101.
4dBDESIGNGUIDELINESResistancemeasuredat1kΩinduces4nV/√Hznoise,whichismorethanthevoltagenoiseofmostoperationalamplifiers.
Inthecircuit,theresistanceoftheresistorsmustbechosencarefullyandmustnotexceed1k.
Shieldingisveryimportantinmobilephones.
Toreach<100dBTHD+Nspecifications,thetiniestofinterferencescandegradeTHD+Nperformance,particularlywhenlisteningtomusicandbrowsingtheinternetsimultaneously.
Metalshieldingcanhelppreventperformancedegradation.
+3.
3V–3.
3V16Ω200Ω200Ω820Ω820ΩSYS-2712BALANCEBNC-UNBALANCE1kHz/1.
0Vrms15032-009–120–100–80–60–40–2000.
020.
2THD+N(dB)OUTPUTVOLTAGE(Vrms)ADA4807-2=16ΩADA4841-2=16ΩAD8397=16Ω15032-010–120–100–80–60–40–200110100THD+N(dB)OUTPUTCURRENT(mA)ADA4807-2=16ΩADA4841-2=16ΩAD8397=16Ω15032-011AN-1429ApplicationNoteRev.
0|Page8of8Forbetterheatdissipation,soldertheexposedpadoftheLFCSPpackagetotheboardpadandconnectittoabigsolidcopperplaneattheoppositesideoftheboardbyvias.
Thecopperplanecanbethegroundorpowerplaneoftheboard,butconsulttheADA4841-2,ADA4807-2,andAD8397datasheetstospecifywhichplanetouse.
Usealowdropoutregulator(LDO)asthepowersupplyofoperationalamplifiers.
Placethedecouplingcapacitors(0.
1μFand4.
7μF)neartheoperationalamplifierpowerpins.
ThecapacitorsintheaudiopathmustbeanNP0ceramictype,astheyofferbetterdistortionperformance.
UsethinfilmresistorsforoptimumTHDperformance.
Metalfilmresistorsarealsosuitable,buttypicallycostmore.
SUMMARYTable3liststhedifferentsolutionsanddifferentconsiderationsforvoltageoutputDACorcurrentoutputDAC.
Therearemanythingstoconsiderwhencreatinganexcellentproduct.
Inrealapplications,accordingtodifferentconditions(forexample,powersupply,targetedloadresistance,powerconsumptionbudget,andexpectedperformance),customerscanchoosetherightsolutionanddesignthecircuittoachievepersonalexpectations.
Table3.
RecommendedDifferentSolutionsDACCurrentOutputParameterDACVoltageOutputItoVStageOutputStageLowCostADA4841-2ADA4841-2ADA4841-2HighPerformanceAD8397ADA4807-2,ADA4896-2,ADA4075-2AD8397QualityPerformancebyLowPowerConsumptionADA4807-2ADA4807-2,ADA4896-2,ADA4075-2ADA4807-22016AnalogDevices,Inc.
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