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1、SECTION4HIGHSPEEDSAMPLINGANDHIGHSPEEDADCs,WaltKesterINTRODUCTIONHighspeedADCsareusedinawidevarietyofreal-timeDSPsignal-processingapplications,replacingsystemsthatusedanalogtechniquesalone.Themajorreasonforusingdigitalsignalprocessingare(1)thecostofDSPprocessorshasgonedown,(2)theirspee
2、dandcomputationalpowerhasincreased,and(3)theyarereprogrammable,therebyallowingforsystemperformanceupgradeswithouthardwarechanges.DSPofferssolutionsthatcannotbeachievedintheanalogdomain,i.e.V.32andV.34modems.However,inorderfordigitalsignalprocessingtechniquestobeeffectiveinsolvinganana
3、logsignalprocessingproblem,appropriatecosteffectivehighspeedADCsmustbeavailable.TheADCsmustbetestedandspecifiedinsuchawaythatthedesignengineercanrelatetheADCperformancetospecificsystemrequirements,whichcanbemoredemandingthaniftheywereusedinpurelyanalogsignalprocessingsystems.Inmosthig
4、hspeedsignalprocessingapplications,ACperformanceandwidedynamicrangearemuchmoreimportantthantraditionalDCperformance.ThisrequiresthattheADCmanufacturernotonlydesigntherightADCsbutspecifythemascompletelyaspossibletocoverawidevarietyofapplications.AnotherimportantaspectofintegratingADCsi
5、ntoahighspeedsystemisacompleteunderstandingofthesamplingprocessandthedistortionmechanismswhichultimatelylimitsystemperformance.HighspeedsamplingADCsfirstwereusedininstrumentationandsignalprocessingapplications,wheremuchemphasiswasplacedontime-domainperformance.Whilethisisstillimportan
6、t,applicationsofADCsincommunicationsalsorequirecomprehensivefrequency-domainspecifications.ModernICprocessesalsoallowtheintegrationofmoreanalogfunctionalityintotheADC,suchason-boardreferences,sample-and-holdamplifiers,PGAs,etc.Thismakesthemeasiertouseinasystembyminimizingtheamountofsu
7、pportcircuitryrequired.AnotherdrivingforceinhighspeedADCdevelopmentisthetrendtowardlowerpowerandlowersupplyvoltages.MosthighspeedsamplingADCstodayoperateoneitherdualorsingle5Vsupplies,andthereisincreasinginterestinsingle-supplyconverterswhichwilloperateon3Vorlessforbatterypoweredappli
8、catio
