Circuit layout technique part 4

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1、CircuitLayoutTechniquesAndTips(PartIVofVI)byBonnieC.Baker,MicrochipTechnologyInc.LayoutTechniquesToUseAsTheADCAccuracyAndResolutionIncreaseInitially,analog-to-digitalconverters(ADCs)rosefromananalogparadigmwherealargepercentageofthephysicalsiliconwasanalog.Asdes

2、igntopologiesevolvethisparadigmshiftedtowhereslowerspeedADCswerepredominatelydigital.Evenwiththison-chipshiftfromanalogtodigitalthePCBlayoutpracticeshavenotchanged.Now,asalways,whenthelayoutdesignerisworkingwithmixed-signalcircuits,keylayoutknowledgeisstillneede

3、dinordertoimplementiteffectively.ThisarticlewilllookatthePCBlayoutstrategiesrequiredforADCsusingsuccessiveapproximationregister(SAR)andSigma-Deltatopologies.SARConverterLayoutSARADCscanbefoundwith8-,10-,12-,16-bitandsometimes18-bitresolutions.Originally,theproce

4、ssandarchitecturefortheseconverterswasbipolarwithR-2Rladders.ButrecentlythesedeviceshavemigratedtoaCMOSprocesswithacapacitive-chargedistributiontopology.Needlesstosaythesystemlayoutstrategyfortheseconvertershasnotchangedwiththismigration.Thebasicapproachtolayout

5、isconsistentexceptforhigher-resolutiondevices.Thesedevicesrequiremoreattentiontopreventdigitalfeedbackfromtheserial,orparallel,outputinterfaceoftheconverter.TheSARconverterispredominatelyanalogintermsofcircuitryandtheamountofrealestatededicatedtothedifferentdoma

6、insonthechip(seeFig.1.)Fig.1:12-bitCMOSSARADCUsesChargeDistributionAcrossACapacitiveArrayWithinthisblockdiagramthesampleandhold,comparator,mostoftheDACand12-bitSARareanalog.Theremainingportionsofthecircuitaredigital.Asaconsequence,mostofthepowerandcurrentneededf

7、orthisconverterisusedfortheinternalanalogcircuitry.ThereisverylittledigitalcurrentcomingfromthedevicewiththeexceptionofthesmallamountofswitchingthatoccursintheDACandatthedigitalinterface.Thesetypesofconverterscanhaveseveralpinsforthegroundandpowerconnections.The

8、pinnamesareoftenmisleadinginthattheanaloganddigitalconnectionscanbedifferentiatedwiththepinlabel.TheselabelsarenotmeanttodescribethesystemconnectionstothePCB,butrathe