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1、HARDWAREDESIGNTECHNIQUESGROUNDINGINMIXEDSIGNALSYSTEMSWaltKester,JamesBryantToday'ssignalprocessingsystemsgenerallyrequiremixed-signaldevicessuchasanalog-to-digitalconverters(ADCs)anddigital-to-analogconverters(DACs)aswellasfastdigitalsignalprocessors(DSPs).Requirements
2、forprocessinganalogsignalshavingwidedynamicrangesincreasestheimportanceofhighperformanceADCsandDACs.Maintainingwidedynamicrangewithlownoiseinhostiledigitalenvironmentsisdependentuponusinggoodhigh-speedcircuitdesigntechniquesincludingpropersignalrouting,decoupling,andgr
3、ounding.Inthepast,"highprecision,low-speed"circuitshavegenerallybeenvieweddifferentlythanso-called"high-speed"circuits.WithrespecttoADCsandDACs,thesampling(orupdate)frequencyhasgenerallybeenusedasthedistinguishingspeedcriteria.However,thefollowingtwoexamplesshowthatinp
4、ractice,mostoftoday'ssignalprocessingICsarereally"high-speed,"andmustthereforebetreatedassuchinordertomaintainhighperformance.ThisiscertainlytrueofDSPs,andalsotrueofADCsandDACs.AllsamplingADCs(ADCswithaninternalsample-and-holdcircuit)suitableforsignalprocessingapplicat
5、ionsoperatewithrelativelyhighspeedclockswithfastriseandfalltimes(generallyafewnanoseconds)andmustbetreatedashighspeeddevices,eventhoughthroughputratesmayappearlow.Forexample,the12-bitAD7892successiveapproximation(SAR)ADCoperatesonan8MHzinternalclock,whilethesamplingrat
6、eisonly600kSPS.Sigma-delta(S-D)ADCsalsorequirehighspeedclocksbecauseoftheirhighoversamplingratios.TheAD772216-bitADChasanoutputdatarate(effectivesamplingrate)of195kSPS,butactuallysamplestheinputsignalat12.5MSPS(64-timesoversampling).Evenhighresolution,so-called"lowfreq
7、uency"S-DindustrialmeasurementADCs(havingthroughputsof10Hzto7.5kHz)operateon5MHzorhigherclocksandofferresolutionto24-bits(forexample,theAnalogDevicesAD7730andAD7731).Tofurthercomplicatetheissue,mixed-signalICshavebothanaloganddigitalports,andbecauseofthis,muchconfusion
8、hasresultedwithrespecttopropergroundingtechniques.Digitalandanalogdesignengineerstendtoviewthesedevicesfromdifferentp
