模拟数字转换器.docx
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模拟数字转换器.docx
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模拟数字转换器
Ananalog-to-digitalconverter
Ananalog-to-digitalconverter(ADC)isusedtoconvertacontinuouslyvariablesignaltoacorrespondingdigitalformwhichcantakeanyoneofafixednumberofpossiblebinaryvalues.Iftheoutputofthetransducerdoesnotvarycontinuously,noADCisnecessary.Inthiscasethesignalconditioningsectionmustconverttheincomingsignaltoaformwhichcanbeconnecteddirectlytothenextpartoftheinterface,theinput/outputsectionofthemicrocomputeritself.
TheI/Osectionconvertsdigital“on/off”voltagesignalstoaformwhichcanbepresentedtotheprocessorviathesystembuses.Herethestateofeachinputline,whetheritis“on”or“off”,isindicatedbyacorresponding“1”or“0”.Intheanaloginputswhichhavebeenconvertedtodigitalform,thepatternsofonesandzerosintheinternalrepresentationwillformbinarynumberscorrespondingtothequantitybeingconverted.
FeedbackControl
Theclassofcontrolproblemstobeexaminedhereisoneofconsiderableengineeringinterest.Weshallconsidersystemswithseveralinputs,someknownascontrolsbecausetheymaybemanipulatedandotherscalledexternaldisturbances,whicharequiteunpredictable.Forexample,inanindustrialfurnacewemayconsiderthefuelflow,theambienttemperature,andtheloadingofmaterialintothefurnacetobeinputs.Ofthese,thefuelflowisaccessibleandcanreadilybecontrolled,whilethelattertwoareusuallyunpredictabledisturbances.
Insuchsituations,oneaspectofthecontrolproblemsistodeterminehowthecontrolsshouldbemanipulatedsoastocounteracttheeffectsoftheexternaldisturbancesonthestateofthesystem.Onepossibleapproachtothesolutionofthisproblemistouseacontinuousmeasurementofthedisturbances,andfromthisandtheknownsystemequationstodeterminewhatthecontrolinputsshouldbeasfunctionsoftimetogiveappropriatecontrolofthesystemstate.
DigitalInterfaceCircuits
Thesignalsusedwithinmicrocomputercircuitsarealmostalwaystoosmalltobeconnecteddirectlytothe“outsideworld”andsomekindofinterfacemustbeusedtotranslatethemtoamoreappropriateform.Thedesignofsectionofinterfacecircuitsisoneofthemostimportanttasksfacingtheengineerwishingtoapplymicrocomputers.Wehaveseenthatinmicrocomputersinformationisrepresentedasdiscretepatternsofbits;thisdigitalformismostusefulwhenthemicrocomputeristobeconnectedtoequipmentwhichcanonlybeswitchedonoroff,whereeachbitmightrepresentthestateofaswitchoractuator.
Caremustbetakenwhenconnectinglogiccircuitstoensurethattheirlogiclevelsandcurrentratingsarecompatible.Theoutputvoltagesproducedbyalogiccircuitarenormallyspecifiedintermsofworstcasevalueswhensourcingorsinkingthemaximumratedcurrents.ThusVohistheguaranteedminimum“high”voltagewhensourcingthemaximumrated“high”outputcurrentIoh,whileVolistheguaranteedminimum“low”outputvoltagewhensinkingthemaximumrated“low”outputcurrentIol.Therearecorrespondingspecificationforlogicinputswhichspecifytheminimuminputvoltagewhichwillberecognizedasalogic“high”stateVih,andthemaximuminputvoltagewhichwillberegardedasalogic“low”stateVil.
Forinputinterface,perhapsthemainproblemfacingthedesigneristhatofelectricalnoise.Smallnoisessignalsmaycausethesystemtomalfunction,whilelargeramountsofnoisecanpermanentlydamageit.Thedesignermustbeawareofthesedangersfromtheoutset.Therearemanymethodstoprotectinterfacecircuitsandmicrocomputerfromvariouskindsofnoise.Followingaresomeexample:
1.Inputandoutputelectricalisolationbetweenthemicrocomputersystemandexternaldevicesusinganopto-isolatororatransformer.
2.Removinghighfrequencynoisepulsesbyalow-passfilterandSchmitt-trigger.
3.Protectingagainstexcessiveinputvoltagesusingapairofdiodestopowersupplyreversiblybiasedinnormaldirection.
Foroutputinterface,parametersVoh,Vol,IohandIolofalogicdeviceareusuallymuchtolowtoallowloadstobeconnecteddirectly,andinpracticeanexternalcircuitmustbeconnectedtoamplifythecurrentandvoltagetodriveaload.AlthoughseveraltypesofsemiconductordevicesarenowavailableforcontrollingDCandACpowersuptomanykilowatts,therearetwobasicwaysinwhichaswitchcanbeconnectedtoaloadtocontrolit.
Withseriesconnection,theswitchallowscurrenttoflowthroughtheloadwhenclosed,whilewithshuntconnectionclosingtheswitchallowscurrenttobypasstheload.Bothconnectionsareusefulinlow-powercircuits,butonlytheseriesconnectioncanbeusedinhigh-powercircuitsbecauseofthepowerwastedintheseriesresistorR..
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