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1、CircuitLayoutTechniquesAndTips(PartIIIofVI)byBonnieC.BakerandEzanaHaile,MicrochipTechnologyInc.ThemajorclassesofparasiticgeneratedbythePCboardlayoutcomeintheformofresistors,capacitorsandinductors.Forinstance,PCBresistorsareformedasaresultoftracesfromcomponentt
2、ocomponent,unintentionalcapacitorscanbebuiltintotheboardwithtraces,solderingpadsandparalleltraces,andcircumstancesthatsurroundwhereinductorsarebuiltcomeintheformofloopinductance,mutualinductanceandvias.Alloftheseparasiticsstandachanceofinterferingwiththeeffect
3、ivenessofyourcircuitasyoutransitionfromthecircuitdiagramtotheactualPCB.Thisarticlequantifiesthemosttroublesomeclassofboardparasitics,theboardcapacitor,andgivesanexampleofwheretheeffectsoncircuitperformancecanbeclearlyseen.FeelingthePainofThoseUnnecessaryCapaci
4、torsInPartIIofthisserieswediscussedhowcapacitorscouldinadvertentlybebuiltintoyourboardand,quicklyreviewingthisconcept,mostlayoutcapacitorsarebuiltbyplacingtwoparalleltracesclosetogether.ThevalueofthistypeofcapacitorcanbecalculatedusingtheformulaeshowninFig.1(f
5、romFig.5inPartIIofthisseries.)Fig.1:CapacitorscaneasilybefabricatedintoaPCBbylayingouttwotracesincloseproximity.Withthistypeofcapacitor,fastvoltagechangesononetracecaninitiateacurrentsignalintheothertraceThistypeofcapacitorcancauseproblemsinmixedsignalcircuits
6、wheresensitive,highimpedanceanalogtracesareincloseproximitytodigitaltraces.Forexample,thecircuitinFig.2hasthepotentialtohavethistypeofproblem.Fig.2:A16-bitDACcanbebuiltusingthree8-bitdigitalpotentiometersandthreeamplifierstoprovide65,536differentoutputvoltages
7、.IfVDDis5VinthissystemtheresolutionorLSBsizeofthisDACis76.3µVToquicklyexplainthecircuitoperationinFig.2,a16-bitDACisbuiltusingthree8-bitdigitalpotentiometersandthreeCMOSoperationalamplifiers.Totheleftsideofthisfigure,twodigitalpotentiometers(U3aandU3b)spanacro
8、ssVDDtogroundwiththewiperoutputconnectedtothenon-invertinginputoftwoamplifiers(U4aandU4b).Thedigitalpotentiometers,U2andU3areprogrammedusinganSPIinterfacebetweenthemicrocontroller,
