ApplicationReportSPRAA88A–September2008UsingPWMOutputasaDigital-to-AnalogConverteronaTMS320F280xDigitalSignalControllerDavidM.AlterDSPApplications–SemiconductorGroupABSTRACTThisapplicationreportpresentsamethodforutilizingtheon-chippulsewidthmodulated(PWM)signalgeneratorsontheTMS320F280x™familyofdigitalsignalcontrollersasadigital-to-analogconverter(DAC).Themethodinvolvesanaloglow-passfilteringthePWMsignaltoremovehighfrequencycomponents,leavingonlythelow-frequencycontent.Theoreticalandexperimentalresultsarepresentedtoquantifytheachievablebitresolutionandbandwidth,whichdependupontheanaloglow-passfilteremployed,thePWMfrequency,andtheDSPoperatingfrequency.WhencoupledwithasecondorderpassiveRLCfilter,thehi-resolutionPWMmoduleontheTMS320F280xdeviceisseencapableofprovidinggreaterthan9bitsofDACresolutionwith100kHzbandwidth.Alternately,50kHzDACbandwidthispossiblewithbetterthan10bitsofresolution.TheseresolutionsandbandwidthsmakethePWM/DACalegitimatelowercostalternativetodedicatedoff-chipDACsinTMS320F280xbasedsystems.™TMS320isatrademarkofTexasInstruments,Inc.Trademarksarethepropertyoftheirrespectiveowners.SPRAA88A2UsingPWMOutputasaDigital-to-AnalogConverteronaTMS320F280xDigitalSignalControllerContents1Introduction.....................................................................................................................................42FrequencyAnalysisofthePWMSignal........................................................................................53D/AResolutionIssues....................................................................................................................64AnalogLow-PassFilters.................................................................................................................74.11stOrderLow-PassFilter...........................................................................................................84.22ndOrderLow-PassFilter..........................................................................................
