PAGE\*MERGEFORMAT10AllAboutDirectDigitalSynthesisWhatisDirectDigitalSynthesis?Directdigitalsynthesis(DDS)isamethodofproducingananalogwaveform—usuallyasinewave—bygeneratingatime-varyingsignalindigitalformandthenperformingadigital-to-analogconversion.BecauseoperationswithinaDDSdeviceareprimarilydigital,itcanofferfastswitchingbetweenoutputfrequencies,finefrequencyresolution,andoperationoverabroadspectrumoffrequencies.Withadvancesindesignandprocesstechnology,today’sDDSdevicesareverycompactanddrawlittlepower.Whywouldoneuseadirectdigitalsynthesizer(DDS)?Aren’tthereothermethodsforeasilygeneratingfrequencies?Theabilitytoaccuratelyproduceandcontrolwaveformsofvariousfrequenciesandprofileshasbecomeakeyrequirementcommontoanumberofindustries.Whetherprovidingagilesourcesoflow-phase-noisevariable-frequencieswithgoodspuriousperformanceforcommunications,orsimplygeneratingafrequencystimulusinindustrialorbiomedicaltestequipmentapplications,convenience,compactness,andlowcostareimportantdesignconsiderations.Figure1.TheAD9833-aone-chipwaveformgenerator.Manypossibilitiesforfrequencygenerationareopentoadesigner,rangingfromphase-locked-loop(PLL)-basedtechniquesforveryhigh-frequencysynthesis,todynamicprogrammingofdigital-to-analogconverter(DAC)outputstogeneratearbitrarywaveformsatlowerfrequencies.ButtheDDStechniqueisrapidlygainingacceptanceforsolvingfrequency-(orwaveform)generationrequirementsinbothcommunicationsandindustrialapplicationsbecausesingle-chipICdevicescangenerateprogrammableanalogoutputwaveformssimplyandwithhighresolutionandaccuracy.Furthermore,thecontinualimprovementsinbothprocesstechnologyanddesignhaveresultedincostandpowerconsumptionlevelsthatwerepreviouslyunthinkablylow.Forexample,theAD9833,aDDS-basedprogrammablewaveformgenerator(Figure1),operatingat5.5Vwitha25-MHzclock,consumesamaximumpowerof30milliwatts.WhatarethemainbenefitsofusingaDDS?DDSdevicesliketheAD9833areprogrammedthroughahighspeedserialperipheral-interface(SPI),andneedonlyanexternalclocktogeneratesi