TIDesigns

HighDynamic-RangeHeadphoneDriverReferenceDesign

forVoltage-Output,DifferentialAudioDACs

TIDesignsDesignFeatures

TIDesignsprovidethefoundationthatyouneed?FeaturesaHigh-Performance,Low-Distortion,

includingmethodology,testinganddesignfilestoAudio-AmplifierSignalChain

quicklyevaluateandcustomizethesystem.TIDesigns?ExploresVariousTrade-offstoExtractOptimum

helpyouaccelerateyourtimetomarket.PerformanceBasedonSystemNeeds

DesignResources

TIDA-00715

ToolFolderContainingDesignFiles

?IncludesLowDistortion,Fully-DifferentialStageto

AmplifySignalFromVoltageOutAudioDACSuch

asPCM1791A

OPA2836ProductFolder

THS4521ProductFolder

OPA1632ProductFolder

?FeaturesaDifferentialtoSingle-Ended2ndStage

AmplifierUtilizingLoadSharingtoDrive

HeadphoneLoad

TINA-TI?ToolsFolder

?CanbeExtendedtoCurrentOutputAudioDACs

SuchasPCM1792AbyConfiguringFDAasa

TransimpedanceAmplifier

ASKOurE2EExperts

WEBENCH?CalculatorTools

FeaturedApplications

?HeadphoneDriverforVoltage-OutputAudioDAC

FullyDifferentialAmplifierDifferentialtoSingle-endedDriver

RF1

R2

RG1R1

+

V

OCM

-

-

+

RS1

RG1'

R1'

R2'

RF1'

R4

R3

-

RS1'

+

R3'

R2'

AnIMPORTANTNOTICEattheendofthisTIreferencedesignaddressesauthorizeduse,intellectualpropertymattersandother

importantdisclaimersandinformation.

SennheiserisatrademarkofTexasInstruments.

Allothertrademarksarethepropertyoftheirrespectiveowners.

TIDUAG7–September2015HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-

SubmitDocumentationFeedback

Output,DifferentialAudioDACs

Copyright?2015,TexasInstrumentsIncorporated

1

KeySystemSpecifications

1KeySystemSpecifications

PARAMETERSPECIFICATION

SupplyVoltage±2.5-Vbipolarsupplies

QuiescentCurrent6mA.Thisincludesbothchannelsonthestereoboard.

Low-PowerMode42μA.Thisincludesbothchannelsonthestereoboard.

MaximumDACOutputCommonMode1.8V

TotalSystemGain1V/V(0dB)

TargetHeadphoneLoad32Ω

TargetOutputLoad30mW

THD+N0.0003%,20-Hzto20-kHzmeasurementbandwidth,A-weighted

IdleChannelNoise2μVRMS(114dBV),20-Hzto20-kHzmeasurementbandwidth,A-weighted

2HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-TIDUAG7–September2015

Output,DifferentialAudioDACsSubmitDocumentationFeedback

Copyright?2015,TexasInstrumentsIncorporated

SystemDescription

2SystemDescription

Figure1showsthecompleteschematicofthelaboratorysetupandtheaudio-amplifiersignalchain.The

outputofthedigital-to-analogconverter(DAC)ismodeledasadifferentialvoltagewitheachoutputhaving

40-?outputimpedance.TheoutputoftheDACdrivestheOPA1632level-shiftcircuitthataltersthe

common-modeoffsetofthedifferentialDACsignal.Thelevel-shiftcircuitdrivestheaudio-amplifierboard,

whichhastwocopiesoftheamplifiercircuit,showninFigure1,forevaluationofstereosignals.Theboard

isequippedwithaheadphonejacktoevaluatetheperformancewithdifferentheadphones.

Voltage-out,audioDACmodelAmplifiercircuit(±2.5-Vsupply)

AudioPrecision?

SYS2522

±AnalogO/P

40

40

360

VOCM

360

400

+

-

400

RO1

40

40

RO1'

RG1

VOCM1

RG1'

RF1

+

-

RF1'

R1

R1'

R2

+

R2'

RS1

OPA1632±

Common-modelevel

shiftcircuit

R3

R4

+

RS1'

AudioPrecision?

SYS2522±AnalogI/P

R3'

R4'

THS4521±

Differentialamplifier

OPA2836±

Headphonedriver

Figure1.BlockDiagramofSystemUsedDuringEvaluation

2.1OPA1632Level-ShiftCircuit

TheAudioPrecision2522cannotproduceadifferentialoutputsignalwithanadjustablecommon-mode

voltage.Instead,afullydifferentialamplifierinaunity-gainconfigurationisusedtolevel-shiftthecommon-

modevoltage.TheadjustablelevelshiftisneededbecauseaudioDACsdifferintheiroutputcommon-

modevoltages.

TheOPA1632waschosenfortheadjustablelevel-shiftbecauseofitsexcellentaudiocharacteristicsand

itsabilitytoreproducethesignalfromtheAudioPrecision2522withnegligiblesignaldegradation.TheOPA1632isoperatedon±15-Vsuppliestomaximizeheadroomandminimizedistortion.TheVOCMpinis

connectedtoaninternalresistor-dividercircuit,whichdefaultstomid-supplywhenleftfloating.Tovarythe

outputcommonmodefromtheOPA1632,itspowersupplyrailsareshiftedasymmetricallyaround0V.

TheoutputcommonmodeoftheOPA1632isalwaysatmid-supplyforoptimumperformance.ThefeedbackresistorsoftheOPA1632aresetto400?asrecommendedinthedatasheet.ResistorsRO1

(andRO1’)inFigure1aresetto40?tomimictheoutputresistanceoftheAudioPrecisionanalogoutputs.

MimickingtheoutputsinsertstheOPA1632intothesignalchainwithoutalteringthetotalcircuitgain.

TIDUAG7–September2015HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-

SubmitDocumentationFeedback

Output,DifferentialAudioDACs

Copyright?2015,TexasInstrumentsIncorporated

3

SystemDescription

2.2THS4521DifferentialAmplifierStage

TheTHS4521amplifiestheoutputoftheaudioDACandalsoshiftsthecommon-modevoltagetoground

forthesubsequentheadphonedriverstage.Analternativeistousetwoparallelsingle-endedamplifiers,

butafullydifferentialamplifier(FDA)hasseveralperformanceadvantagesincluding:

?SuperiorHD2performance.ThearchitectureoftheFDAprovidesinherentHD2performancebenefits

comparedtoitssingle-endedopampcounterparts.Inaddition,eachsingle-endedoutputoftheFDA

onlyneedshalfthevoltageswingtoproducethesamedifferentialoutputswingasasingle-ended

amplifier.

?Separatecommon-modeanddifferentialgains-Thedifferentialgain,incaseofafully-differentialinput,

is–RF/RG.Theoutputcommon-modevoltageisindependentlycontrolledwiththeVOCMpin,makingit

independentofthecommon-modevoltageoftheinputandalsoofthedifferentialgain.

?Improvednoiseperformance.AnynoisefromGNDorthepowersuppliesisautomaticallyrejectedby

theFDAduetoitsdifferentialarchitecture.

MoreinformationonFDAscanbefoundinFully-DifferentialAmplifiersApplicationNote(SLOA054D).Figure2showstheblockdiagram.

RF

R

G

RG'

+2.5V

VOCM

-2.5V

+

-

RF'

Figure2.THS4521FDABlockDiagram

4HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-TIDUAG7–September2015

Output,DifferentialAudioDACsSubmitDocumentationFeedback

Copyright?2015,TexasInstrumentsIncorporated

SystemDescription

2.3OPA2836HeadphoneDriverStage

ThefunctionoftheOPA2836istwo-fold:

1.ConvertthedifferentialoutputsignalfromtheTHS4521toasingle-endedsignal.

2.Drivetheheadphoneload.

TheOPA2836isavailableinadual-channel2×2-mmpackagethatfacilitatesanextremelycompact

designforportableapplications.Thedevicealsofeaturesanexcellentperformance-to-powerratio,making

itanidealchoiceforhighdynamic-rangeaudioapplications.

R2

R1

+2.5V

RS1

+

R1'

R2'

-2.5V

R4

RLOAD

R3

+2.5V

RS1'

+

R3'

R4'

-2.5V

Figure3.OPA2836DriverStage

Table1.KeySpecificationsofOPA2836andTHS4521

QUIESCENTCURRENTGBWBROADBANDNOISEHD2/HD3at100kHz

DEVICE(mA)(MHz)SLEWRATE(V/μs)(nV/√Hz)(dBc)

OPA28361.01185604.6–133/–140

THS45211.0934904.6–133/–141

TIDUAG7–September2015HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-

SubmitDocumentationFeedback

Output,DifferentialAudioDACs

Copyright?2015,TexasInstrumentsIncorporated

5

SystemDescription

2.4SystemDesignOptionsandTradeoffs

ForthespecificationslistedinTable1,theOPA2836andTHS4521arequitesimilar,whichcreates

interestingdesigntrade-offconsiderations.Thetargetedsignalchaingainis1V/V,whichcanbeachieved

inthreeways:

?GainofTHS4521=1andGainofOPA2836=1

?GainofTHS4521>1andGainofOPA2836=1/(GainofTHS4521)

?GainofTHS4521<1andGainofOPA2836-1/(GainofTHS4521)

Theoutputnoiseofacascadedbufferstageisgivenby

22

V=(A′(A′N1))+(A′N2)

N_OUTVS2VN1VN2

,whereAVN1andAVN2arethenoisegainsofeachstage,N1andN2arethevoltage-noisedensitiesofeachstage,andAVS2isthesignalgainofthesecondstage.

Fromtheequation,thegainshouldbeconcentratedinthefirststageasthetotalnoisefrombothstagesisamplifiedbythegainofthesecondstage.TheoverallnoisedecreasesasAVS2isreduced.Thesecond

optioninthepreviouslistwillprovidethelowestnoise.

N1N2

AVN1

AVN2

AVS2

Figure4.CascadedBufferStage

Bothamplifiershavesimilardistortionspecifications,sothefirsttwooptionsyieldsimilardistortion

performance.Inthisapplication,theOPA2836isdrivingtheheavyheadphoneload,whiletheTHS4521is

drivingamuchlighterloadformedbytheinputresistorsofthesecondstage.Theheavyheadphoneload

willdegradethedistortionperformanceoftheOPA2836.Thearchitectureofthesecondoptionisthe

betterchoice,asitallowsconfigurationoftheOPA2836withincreasedloopgaincomparedtothefirst

option.Whilethesecondoptionappearstobethebetterchoice,itisimportanttocarefullydeterminethe

optimumgainofeachstage.Increasingthegainofthefirststagetoomuchdegradesperformancedueto

swinglimitationsandreducedloopgainintheFDA.TheTHD+Nperformanceofbothoptionsconfirms

thatthesecondoptionisbetter.

TheOPA2836hasamaximumlinearoutputcurrentdriveof±45mA,sothedevicecandrivea600-Ω

headphoneloadwithease.However,tosharetheloadwhendriving32-Ωand16-Ω,thesecondchannel

intheOPA2836packageisconnectedinparallelwiththefirstchannel.

ThemaximumoffsetvoltageoftheOPA2836isspecifiedas±400μV.Iftheoutputsofthetwoamplifiers

aredirectlyconnected,thedifferentialoffsetvoltagebetweentheamplifierscausesashortandprevents

theamplifiersfromoperatingnormally.Topreventthesignalsfromshorting,asmallseriesresistanceRS1=RS1’=1Ωisinsertedattheoutputoftheeachamplifier.Theresistanceservestolimitthecurrent

flowbetweentheamplifieroutputs.Withtheseresistorsinplace,theworst-caseDCcurrentbetweenthe

amplifieroutputsis(800μV?2Ω)=400μA.

Anaudiojackontheboardenablestestingwithheadphones;however,theresultsshownbeforeSection5.3arewitha32.4-?resistiveload.

6HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-TIDUAG7–September2015

Output,DifferentialAudioDACsSubmitDocumentationFeedback

Copyright?2015,TexasInstrumentsIncorporated

Simulations

Tovalidatetheseconclusionsaboutthegainofeachstage,twoconfigurationswerechosenforfurther

evaluation.ThevaluesinTable2werechosenbasedonthecomponentsinFigure1.

Table2.ResistanceValuesforEachGainConfiguration

GAIN

CONFIGURATIONRRF1(andRF1’)

G1(andRG1’)Ω(1)R1andR3(R1’andR3’)ΩR2andR4(R2’andR4’)Ω

(GTHS4521-GOPA2836)Ω

(V/V)

1.50-0.6769811101110741

1.00-1.001110111011101110

(1)Thereisanadditional40Ωofresistancefromthepreviousstage:OPA1632circuitorAudioPrecision

Theresistorsusedinthisdesignhaveatoleranceof0.1%thatfacilitatescancellationofeven-order

harmonicdistortionintheFDAstage.Thedesignoftheresistorsalsomaximizesthecommon-mode

rejectioninthedifferential-to-single-endedamplifierstage.Whereverpossible,thecapacitorsusedinthe

referencedesignareoftheC0G/N0Ptypeceramic.Thesecapacitorshavesmallvoltagecoefficients,

whichreducesdistortionwhenlargevoltagesareappliedacrossthem.Moreinformationcanbefoundin

SignalDistrotionfromHigh-KCeramicCapacitorsandMoreAboutUnderstandingtheDistortion

MechanismofHigh-KMLCCs.

3Simulations

TheschematicshowninFigure5isusedtosimulatenoiseandverifytheDCandtransientbehaviorofthe

audiocircuit.Twovoltage-controlledvoltagesources(VCVs)areusedtoemulatethesourceofAudio

Precision.

1.50-0.67V/VGainConfig.HeadphoneDriver

R11741

DACOuputChannelLevelShiftCircuitDifferentialAmplifier

(FromAudioPrecision)

Vs-

VCVS1500m

R71.11k

++

U5OPA1632

U3THS4521

R131

R17400R31.11k

R61.11k

--

+

+PD

VInp

Vs2-U1OPA2836

R2140

R19360

Vocm

R2240R20360

1

2

8

-

FDA

+

-

+

+

4

5

R1540

R1640

R5701

R1701

1

2

8

Vs-

Vs+

PD

-

4

5

FDA

+

-

+

+

Vs+

Vs+

R4741

Vs-

VG1

+

VOut

-

Vs2+

R18400

R21.11k

R91.11k

++

R141

--

VCVS2500m

Vs2+

Vs2-Vocm

Vs+

Vs-

R101.11k

+PD

+

U4OPA2836

Vs+

V42.5V5-2.5

Figure5.TINASchematicofSystem

TIDUAG7–September2015HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-

SubmitDocumentationFeedback

Output,DifferentialAudioDACs

Copyright?2015,TexasInstrumentsIncorporated

7

Simulations

3.1Noise

Theidle-channelnoiseforbothgainconfigurationswassimulated.TheOPA1632wasexcludedfromthe

systemforthissimulation.TheinputsoftheTHS4521stageweredirectlyconnectedtotheDACoutput,

andthetotalnoisewassimulatedoveranoise-bandwidthof22Hzto22kHz,asshowninFigure6.From

theresultsinFigure6,theidle-channelnoiseforthe1-1V/V(optiona)and1.50-0.67V/V(optionb)configurationswere2.30μVRMSand1.85μVRMS,whichcorrespondsto–112.76and–114.66dBV,

respectively.ThecurrentTINA-TImodelsareunabletosimulatedistortion,soTHD+Nwasonlymeasured

andnotsimulated.

10P

G=1.50-0.67GainConfiguration

G=1.00-1.00GainConfiguration

2.30PVRMS

1P

1.85PVRMS

100n

101001k10k

Frequency(Hz)

D001

Figure6.SimulatedOutputNoiseinBothGainConfigurations

3.2DACOutputCommon-ModeRange

Bothgainconfigurationsweresimulatedwhilevaryingtheinputcommon-modevoltage.Theinput(RG1)

andfeedback(RF1)resistorsoftheFDAstageactasavoltagedividerbetweentheoutputcommon-mode

voltageoftheDACandtheoutputcommon-modevoltageoftheFDA.TheoutputDCoffsetofthe

THS4521issettomid-supply.FromtheTHS4521datasheet,thecommon-modeinputvoltagehighlimit

is3.6Vwhenthesuppliesare0Vand5V,respectively.For±2.5-Vsupplies,thecommon-modeinput

voltagehighlimitis1.1-V.Equation1determinesthemaximumoutputcommon-modevoltageoftheDAC

thatwillnotexceedthemaximuminputcommon-modevoltageoftheTHS4521.

V

CM_MAX(DAC)

=

V(RR)

+

CM_MAX(FDA)FG

R

G

(1)

FromEquation1,Equation2andEquation3themaximumDACcommon-modevoltageforthedifferent

gainconfigurations.

1.1V(1.11k1.11k40)

W+W+W

V2.24V

==

CM,1-1V/Vgain

1.11kW

(2)

1.1V(1.11kW+701kW+40W)

V==1.83V

CM,1.50-0.67V/Vgain

1.11kW

(3)

Thesecalculationsshowthattheinputcommon-moderangeoftheTHS4521isreducedasitsgainis

increased,whichlimitsthemaximumgainsettingoftheTHS4521stage.

8HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-TIDUAG7–September2015

Output,DifferentialAudioDACsSubmitDocumentationFeedback

Copyright?2015,TexasInstrumentsIncorporated

PCBDesign

4PCBDesign

Figure7andFigure8showthePCBlayoutandschematic,respectively.Thebillofmaterials(BOM)can

befoundinTable5.Tomaximizeperformance,minimizetracelengthstoreduceparasitictrace

resistance,andensurethosetracelengthsmatch.

Shutdownswitch

Stereochannel1

Stereochannel2

Activecircuitarea/channel

=165mm2

Figure7.AudioBoardPCB

TIDUAG7–September2015HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-

SubmitDocumentationFeedback

Output,DifferentialAudioDACs

Copyright?2015,TexasInstrumentsIncorporated

9

PCBDesign

4.1Schematic

Figure8showstheAudioBoardCircuitschematic.

C2

DNI

C4

+Vs+Vs-Vs-Vs

VF1

SMA

GND

VF2

SMA

GND

0Ω

C11

0Ω

C12

698Ω

R5

0.1%

R4698Ω

0.1%

+Vs

R41

1.1KΩ

0.1%

DNI

PD1

1

2

8

7

R1

1.1KΩ

0.1%

-Vs

U1

4

V-

VOCM

5

V+

THS4521

+Vs

R2

1.1KΩ

0.1%

C3

DNI

GND

0Ω

R19

0Ω

R18

VF5

DNI

VF4

1

SMA

R45

DNI

R46

DNI

1

SMA

0.1%

R7

1.1KΩ

R3

1.1KΩ

0.1%

PD2A

DNI

R8

732Ω

0.1%

+Vs

4U2A

OPA2836

2

1

3

R6

-Vs

732Ω

0.1%

C6

DNI

C23

C24

C25

C26

1000pF

100pF

1000pF

100pF

GNDGNDGNDGND

VF6

1Ω

SMA

R13

R15

32Ω

GND

GND

AudioJack

J8

2

3

1

SJ-3523-SMT

GND

R42

C5

0.1μF

DNI

GND

R12

732Ω

0.1%

1.1KΩ

0.1%

DNI

-Vs

GND

+Vs+Vs-Vs-Vs

C15

1000pF

C16

100pF

C17

1000pF

C18

100pF

0.1%

R11

1.1KΩ

1.1KΩ

R9

0.1%

PD2B

6

8

7

+Vs

U2B

OPA2836

R14

+Vs

L1

1Ω

9

GNDGNDGNDGND

+Vs

LQM31PN1R5MC0L

-Vs

SPC15363

C39

GND

10μF

C40

0.1μF

R10

732Ω

VF7

SMA

C7

DNI

R21

1.1KΩ

0.1%

-Vs

VF10

DNI

1SMA

R47

DNI

0.1%

C9

DNI

R28

732Ω

0.1%

+Vs

+Vs+Vs-Vs-Vs

C27

C28

C29

C30

1000pF

100pF

1000pF

100pF

GNDGNDGNDGND

SPC15354

-Vs

SPC15182

S1

L2

C41

10μF

LQM31PN1R5MC0L

-Vs

C42GND

0.1μF

GND

VF8

SMA

GND

0Ω

C13

0Ω

C14

R24

698Ω

0.1%

R23

698Ω

0.1%

PD3

+Vs

R43

1.1KΩ

0.1%

DNI

R44

1.1KΩ

0.1%

DNI

-Vs

GND

C1

0.1μF

C19

1000pF

U3

1

2

8

7

C20

100pF

THS4521

V-

4

VOCM

5

V+

+Vs

R22

1.1KΩ

0.1%

C8

DNI

+Vs+Vs-Vs-Vs

C21

1000pF

C22

100pF

GNDGNDGNDGND

GND

0Ω

R36

0Ω

R35

VF11

DNI

1

SMA

GND

R48

DNI

0.1%

R271.1KΩ

1.1KΩ

R25

0.1%

0.1%

R31

1.1KΩ

1.1KΩ

R29

0.1%

PD4A

PD4B

4U4A

OPA2836

2

1

3

-Vs

R26

732Ω

0.1%

C10

DNI

R32

0.1%

732Ω

+Vs

6U4B

8

9

7

OPA2836

1Ω

R33

1Ω

R34

R37

32Ω

GND

VF12

SMA

GND

+Vs

-Vs

3

1

2

CL-SB-12B-01T

R16PD1

49.9Ω

R17PD2A

49.9Ω

R20PD2B

49.9Ω

R38PD3

49.9Ω

R39PD4A

49.9Ω

R40PD4B

49.9Ω

TPG1TPG2TPG3

GND

-Vs

R30

732Ω

0.1%

Figure8.TIDA-00715AudioBoardCircuitSchematic

10HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-TIDUAG7–September2015

Output,DifferentialAudioDACsSubmitDocumentationFeedback

Copyright?2015,TexasInstrumentsIncorporated

VerificationandMeasuredPerformance

5VerificationandMeasuredPerformance

Figure9showsthebenchsetupusedtomeasuretheperformanceoftheaudioboard.Twotriple-output

powersuppliesareusedtopowerboththeOPA1632level-shiftcircuitandtheaudioreferenceboard.A

PCisusedtocontroltheAudioPrecisionAP2522interfaceandretrieveoutputdata.TheAudioPrecision

A-weightingfilteroptionisenabled,andthemeasurementbandwidthissetto20Hzto24kHzforall

measurementsinthissection.

Figure9.LaboratoryBenchSetupShowingAudioPrecision,OPA1632,andAudioReferenceBoard

5.1QuiescentCurrent

Oneofthemainbenefitsinthisamplifierdesignisitspower-to-performanceratio.BoththeTHS4521and

theOPA2836typicallyconsume1.0mAperchannel,resultinginatypicalquiescentpowerconsumption

of15mWforeachchannelofastereoamplifier.Themaximumquiescentpowerperchannelofthestereo

amplifieris18.25mW.BoththeTHS4521andOPA2836canbeplacedinshutdownmodeandconsume

lessthan11.5μAperchannel.Aswitchisincludedontheaudioreferenceboardtocontrolthestateofthe

shutdownpins.

TIDUAG7–September2015HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-

SubmitDocumentationFeedback

Output,DifferentialAudioDACs

Copyright?2015,TexasInstrumentsIncorporated

11

VerificationandMeasuredPerformance

5.2AP2522andOPA1632Level-ShiftCircuitBaselinePerformance

Toestimatetheperformanceoftheaudioboard,baselinemeasurementsoftheAP2522andOPA1632

level-shiftcircuitaremadetoensurethatthesetupisnotthelimitingfactorinthesystemmeasurements.

TheTHD+NbaselinemeasurementisshowninFigure10fora1-kHzsignal.TheresultsinFigure10set

aminimumforbestsystemperformanceexpectedwhentestingwithandwithouttheOPA1632circuit.The

OPA1632doescausesomesignaldegradation.Thex-axisinthefigureisinwatts,referredtoa32-Ω

load.Thereisnoloadconnectedtothesystemforthismeasurement.The32-Ωloadisassumedfor

compatibilitywiththemeasurementsinSection5.3andSection5.4.

1

AP2522

AP2522+OPA1632

0.1

0.01

0.001

0.0001

10n100n1P10P100P1m10m100m

OutputPower(W)

D002

Figure10.BaselineTHD+NPerformanceofAP2522andOPA1632

Figure11showsthebaselineFastFourierTransform(FFT)performancewhendrivinga2-VPPsignalout

oftheAP2522.Thelow-frequencyspursaredueto60-Hznoisefromthesupplies.

0

-10

-20

AP2522

AP2522+OPA1632

-30

-40

-50

-60

-70

-80

-90

-100

-110

-120

-130

-140

-150

101001k10k

Frequency(Hz)

D003

Figure11.1-kHzBaselineFFT,VOUT=2VPP

12HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-TIDUAG7–September2015

Output,DifferentialAudioDACsSubmitDocumentationFeedback

Copyright?2015,TexasInstrumentsIncorporated

VerificationandMeasuredPerformance

5.3AudioBoardPerformanceWith32-ΩResistiveLoad

Measurementsinthissectionaremadewitha32-?resistiveloadwiththeA-weightingfilteroptionoftheAudioPrecisionenabled.Themeasurementbandwidthissetintherangeof20Hzto24kHz.

5.3.1IdleChannelNoise

Table3comparesthesimulatedandmeasuredidle-channelnoiselevelsforbothgainconfigurations.The

valuesforbothcasesarecloselymatched.

Table3.ComparingSimulatedandMeasuredTotalOutputNoise

GAINCONFIGURATIONSIMULATEDIDLECHANNELNOISE(dBV)MEASUREDIDLECHANNEL

(GTHS4521/GOPA2836)(V/V)NOISE(dBV)

1.00–1.00–112.76–113.532

1.50–0.67–114.66–115.605

TIDUAG7–September2015HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-

SubmitDocumentationFeedback

Output,DifferentialAudioDACs

Copyright?2015,TexasInstrumentsIncorporated

13

VerificationandMeasuredPerformance

5.3.2THD+NvsOutputPowerWithVariationinDACOutputCommon-ModeVoltage

Figure12andFigure13showthe1-kHz,THD+Nversusoutputpowerforbothgainconfigurationsasa

functionoftheinputcommon-modevoltagetotheaudioboard.The1-1V/Vgainsetupallowsforawider

inputcommon-moderangeatthecostofTHD+Nperformance.FromSection3.2,thetheoretical

maximumcommon-modevoltageintotheaudioboardforthe1-1V/Vgainconfigurationis2.24V.As

showninFigure12,thecommon-modevoltagecanbeabout40%higherwithminimalchangein

performance.

10

1

VCM=0.00V

VCM=2.44V

VCM=2.97V

VCM=3.12V

0.1

0.01

0.001

0.0001

100n1P10P100P1m10m100m

OutputPower(W)

D004

Figure12.G=1-1V/VConfig,1-kHzIn,THD+NvsOutputPowervs.DACOutputCommon-Mode

Similarly,fromFigure13forthe1.50-0.67V/Vgainconfiguration,thereisminimaldifferenceinTHD+N

performancewhentheinputcommon-moderangeisincreasedby40%beyondthecalculatedlimit.

ComparingFigure12withFigure13,the1-1V/Vgainconfigurationcandelivermorepowertotheloadat

thecostofslightlydegradedTHD+Nperformance,indicatingthattheTHS4521isprobablythelimiting

factortothesystemTHD+Nperformance.Thehigherclosed-loopgainoftheTHS4521inthe1.5-0.67V/V

gainconfigurationexperiencesheadroomlimitationsataloweroutputpowercomparedtothe1-1V/Vgain

configurationwhichdegradestheamplifiersdistortion.Intheregionwithlowoutputpowerwherenoiseis

dominant,oneseesthebenefitofthe1.5-0.67V/Vgainconfiguration.

10

1

VCM=0.00V

VCM=2.00V

VCM=2.22V

VCM=2.57V

0.1

0.01

0.001

0.0001

100n1P10P100P1m10m100m

OutputPower(W)

D005

Figure13.G=1.50-0.67V/VConfig,1kHzIn,THD+NvsOutputPowerDACOutputCommon-Mode

14HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-TIDUAG7–September2015

Output,DifferentialAudioDACsSubmitDocumentationFeedback

Copyright?2015,TexasInstrumentsIncorporated

VerificationandMeasuredPerformance

5.3.3OutputSpectrumvsVariationinDACOutputCommon-ModeVoltage

Figure14andFigure15showthe1-kHzoutputspectraforthetwogainconfigurationsasafunctionofthe

DACoutputcommon-modevoltage.Theoutputpoweris15.6mW(2-Vppoutput)inthesefigures.

Comparingtheseresultswiththeidle-channelnoisemeasurementsinTable3.Comparingtheseresults

withthemeasurementsinTable3,noisedominatestheTHD+Nperformanceinthe1-1V/Vgain

configuration.

Figure14.G=1-1V/V,HDvsDACOutputCommon-ModeVoltage,VOUT=2VPP

Figure15.G=1.5-0.67V/V,HDvsDACOutputCommon-ModeVoltage,VOUT=2VPP

Inthecaseofthe1.5-0.67V/Vgainconfiguration,thenoiseandHD3starttobecomethedominant

factors;butsincethissettinghaslowernoise,theresultantTHD+Nperformanceissuperiortotheformer

gainconfiguration.

TIDUAG7–September2015HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-

SubmitDocumentationFeedback

Output,DifferentialAudioDACs

Copyright?2015,TexasInstrumentsIncorporated

15

VerificationandMeasuredPerformance

TheHD2performancefor1.5-0.67V/Visalsomarkedlysuperiortothatof1-1V/V,indicatingthatthe

OPA2836isthedominantHD2contributor.Thus,goingtoahigherloopgaininthedriverstageimproves

theoverallHDperformance.TheheavyloadthattheOPA2836isdrivingresultsinthepowersupplies

becomingcontaminatedbyHD2components.TheoutputstageoftheOPA2836hasaclass-AB

configuration;hence,duringeachhalf-cycleofasine-wave,onesideoftheoutputstageisprovidingallof

thecurrent,inducingahalf-waverectifiedloadcurrentintheoutputstageoftheamplifier,whichdegrades

theHD2.HD3performanceislimitedbytheAP2522+OPA1632asshowninFigure11.

5.3.4Pop-and-ClickResponse

Pop-and-clickreferstospurioustransientsignalproducedattheoutputoftheheadphonewhentheaudio

signalchainiseitherenabledordisabled.Largepop-and-clicktransientscouldunderminethecommercial

attractivenessoftheproductbecauseitcouldcauseirritationtotheenduser.Theaudioboardisequipped

withaswitchthatcontrolsthePowerDown(PD)pinoftheTHS4521andOPA2836.Theswitchwasused

toenableanddisabletheaudioboardamplifiersinordertoevaluatethepop-and-clickresponse.

Figure16showsthepop-and-clickresponsesoftheaudioboardwiththeaudioboardinputsgrounded

andwiththeA-weightingfilterapplied.Glitchesofthismagnitudewillnotcauseanyaudibleartifacts

duringenablinganddisablingevenforhigh-endheadsets.

350

300

250

DisableResponse

EnableResponse

200

150

100

50

0

-50

-100

-150

-200

-250

-300

-350

0102030405060708090

Time(ms)

D006

Figure16.Pop-and-ClickResponseinG=1.5-0.67V/VConfiguration

16HighDynamic-RangeHeadphoneDriverReferenceDesignforVoltage-TIDUAG7–September2015

Output,DifferentialAudioDACsSubmitDocumentationFeedback

Copyright?2015,TexasInstrumentsIncorporated

VerificationandMeasuredPerformance

5.3.5MultiplexedOperation

Incertainapplications(forexample,portablewithsupportforHiFiaudio),havingtwoseperatesignalpaths

may