illdependonlyonthedistancedbetweentheelectrodes.Ⅲ.ELIMINATIONOFPARASITICCAPACITANCESBesidesthedesiredsensorcapacitanceC,therearealsomanyparasiticcapacitancesintheactualstructure(Fig.).ThesecapacitancescanbemodeledasshowninFig..HereCplrepresentstheparasiticcapacitancesfromtheelectrodeEandCpfromtheelectrodeEtotheguardelectrodesandtheshielding.ParasiticcapacitanceCpresultsfromimperfectshieldingandformsanoffsetcapacitance.WhenthetransducercapacitanceCxisconnectedtoanACvoltagesourceandthecurrentthroughtheelectrodeismeasured,CplandCpwillbeeliminated.Cpcanbeeliminatedbyperforminganoffsetmeasurement.Fig..EliminationofparasiticcapacitancesThecurrentismeasuredbytheamplifierwithshuntfeedback,whichhasaverylowinputimpedance.Toobtaintherequiredlinearity,theunity-gainbandwidthfToftheamplifierhastosatisfythefollowingcondition:pffTCCCTf()了理想传感器电容Cx,在实际结构中还有许多寄生电容(图)。