模拟数字转换器.docx

模拟数字转换器.docx

《模拟数字转换器.docx》由会员分享，可在线阅读，更多相关《模拟数字转换器.docx（9页珍藏版）》请在冰豆网上搜索。

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..

　　AT89C51

　　CompatiblewithMCS-51?

Products

　　8KB