Capacitive Sensing: Ins and Outs of Active …

1 ApplicationReportSNOA926A February2015 RevisedFebruary2015 CapacitiveSensing:Ins and Outsof ActiveShieldingDavidWangABSTRACTC apacitivesensinghasbecomea popularalternativeto opticalsensingdueto the advantagesitpossessesin proximity/gesturedetection,materialanaly sis,liquid-levelsensing, powerconsumption,sensingdifferenttypesof materials,and sensingrange,thereare certainaspectsof usingthe technologythat ,parasiticcapacitance,externalinterferen ce,andtemperature/humiditychangesshouldb eaddressedin ,thereare waysto helpmitigatethesefactorsso it doesnotaffectthe thosewaysis reduceEMI interferenceand helpfocusthe sensingfield of a conceptof activeshielding,use casesfor howshieldingcan helpmitigateparasiticand environmentalinterferencefactors,and how to implementit with the FDC1004in ActiveShieldingin of Figures1 ElectricFieldLinesBetweenCH and GND- no DifferentialConfiguration, Top SHLD1 RingaroundCIN1with ProximityFromthe Top ProximityFromthe a registeredtrademarkof othertrademarksare the propertyof February2015 RevisedFebruary2015 CapacitiveSensing:Ins and Outsof ActiveShieldingSubmitDocumentationFeedba ckCopyright 2015,TexasInstrumentsIncorporatedDensity Plot: V, : > + : : : : : : : : : : : : : : : : : : < +000 : Plot: V, : > + : : : : : : : : : : : : : : : : : : < +000.

2 (ch-gnd) fieldE(ch-gnd) fieldchgndchgndshieldNo ShieldShieldedSensor detects objects above and below sensorSensor detects objects only sourceof interferencein capacitivesensingis parasiticcapacitanceto groundalongthe signalpathbetweenthe sensingdevicepins and the interferingcapacitancecan berepresentedby any part of the circuit,or a physicalobject,that causesan unintendedalterationin simpleway to mitigatethis problemis an shielddriveris an activesignaloutputthat isdrivenat the samevoltagepotentialof the sensorinputso thereis no potentialdifferencebetweenthe shieldand externalinterferencewill coupleto the shieldelectrodewithminimalinteractionwit h the severalbenefitsto usinga shieldin shield: Directsand focusesthe sensingzoneto a particulararea Reducesenvironmentalinterferers Reducesand eliminatesparasiticcapacitances Eliminatestemperaturevariationeffectson the followingsectionsaddressthesebenefitsin ActiveShieldingin ShieldFigure1 displaysthe electricfield linesbetweena channelelectrode(CH)and a groundelectrode(GND)in a is excitedwith a voltagewaveformwhilethe GNDis a electricfield lines(red lines)startfromthe highervoltagepotentialelectrode(CH)and endat the lowervoltagepotentialelectrode(GND)

3 ,and are symmetricaboveand belowthe shield,the sensorCH detectsobjectsaboveand belowthe sensorand dependingon theapplication,detectionaboveand belowmay not be acceptableand can misrepresentthe capacitancemeasurementsrelativeto a usinga shieldsensorunderneaththe CH and GNDelectrodes,thefield linesbeloware essentiallyblockedand only the top field lineshavea exampleissomewhatsimplifiedand doesnot ElectricFieldLinesBetweenCH and GND- no ShieldThe shapeand positionof the shieldrelativeto the sensoris an sensinganglewithouta shieldpicksup any strayinterferencewithinthe sensinganglewith a shieldis dependenton how largethe shieldis comparedto the sensorand how closetheshieldis to the illustrateshow the sensoris focusedby the use of a performedwith 4 differentshieldconfigurationsand observationscan be foundin :Ins and Outsof ActiveShieldingSNOA926A February2015 RevisedFebruary2015 SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporatedFDCC apacitance to Digital ConverterNo ShieldWaterLevel SensorCINE nvironmental InterfererstFDCOUTC orrupted DataFDCC apacitance to Digital ConverterShieldWaterCINE nvironmental InterfererstFDCOUTC lean DataShieldLevel SensorFDCC apacitance to Digital ConverterFDCC apacitance to Digital ConverterCINSENSORS ensing AngleCINS ensing ActiveShieldingin CapacitiveSensingSolutionsFigure2.

4 Direct/Focusingthe shieldwrappedaroundthe signalpathbetweenthe sensorand the inputpin to the device(FDC1004)blocksenvironmentalinterf erersfromaffectingthe humanhand,radiatedelectromagneticsignals ,and shieldhas the samedrivesignalas the sensor,any interferenceis pickedup alongthe shieldline,whilethe sensorline is visuallydemonstrateshow a sensorline Shieldingto MitigateEnvironmentalInterferers3 SNOA926A February2015 RevisedFebruary2015 CapacitiveSensing:Ins and Outsof ActiveShieldingSubmitDocumentationFeedba ckCopyright 2015,TexasInstrumentsIncorporatedNo ShieldCINS hieldFDC1004 VEXCS ensorFR4 Ground PlaneCPARCINL arge Ground Parasitic CapacitanceN Can be > 100 pFCINVEXCS ensorFR4 ShieldCPARFDC1004FR4 Ground PlaneShieldParasitic Ground Capacitance EliminatedBenefitsof ActiveShieldingin capacitivesensingdesignwill mostlikelybe implementedon a rigid printed-circuitboard(PCB).Goodsystem-lev eldesignprinciplesrequirea groundplaneto help reducenoiseand capacitivesensingapplications,a groundplanebecomesan issuebecauseit createsa terminationsourcefor electricfield lineseventhoughthe groundplaneis not wherethe intendedsensingareashouldbe.

5 If the sensoris on the top layerand the groundplaneis on the bottomlayerof a PCB(as showninFigure4), fringingeffectsoccurand causethe capacitancemeasurementsto includethe capacitancepathfromthe sensorto the largegroundparasiticcapacitancecan be reducedsignificantlywith a shieldplanebetweenthe sensorand the idealcase,the shieldeliminatesallinfluencefromthe groundplanebut becauseof fringingeffects,a smallparasiticgroundcapacitanceamountis seenin the shieldsize wouldhaveto be muchgreaterthanthe size of thesensorand groundplaneso the field lineson the edgesare muchweakercomparedto the GroundPlaneEffectsWithand GroundPlanesTemperatureis a factorthat causesthe parasiticgroundplanecapacitanceto vary in additionto the initialparasiticcapacitanceoffsetit introducesinto the seenas drift in theconversionresultand may significantlycompromisethe shieldplanebetweenthe sensorand groundplanewill help mitigatethe influenceof the parasiticgroundplanecapacitancefromthe.

6 Ins and Outsof ActiveShieldingSNOA926A February2015 RevisedFebruary2015 SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporatedS1S2S3S 4 CAPACITANCETO DIGITALCONVERTERI2 CFDC1004 SDASCLVDDGNDOFFSET&GAINCALIBRATIONCONFIG URATION REGISTERS&DATA REGISTERSCAPDACMUXEXCITATIONMUXCIN1 CIN2 CIN3 CIN4 SHLD2 SHLD1 CHACHBNo ShieldCINS hieldFDC1004 VEXCS ensorFR4 Ground Plane2 PAR =f(T)CINT emperature Changes Result in Expansion/Contraction of the Ground PlaneCINVEXCS ensorFR4 ShieldFDC1004FR4 Shield2 PAR =f(T)Ground PlaneCapacitance to Ground EliminatedNo Effects From Temperature Changes on Ground FDC1004 Figure5. TemperatureEffectson GroundPlanes3 TypicalImplementationWiththe FDC1004has four channelsensorinputs(CIN1 4) and two shielddriveroutputs(SHLD1 2). Forsingle-endedmeasurements,SHLD1and SHLD2are internallyshorted(if CAPDACis disabled)and canbe pairedwith any of the four channelsensorinputsshownin Figure6.

7 If the CAPDACis usedwithsingle-endedmeasurements,thenSHL D2is floating,as shownin Shieldingin Single-EndedConfigurationWithCAPDACD isabled5 SNOA926A February2015 RevisedFebruary2015 CapacitiveSensing:Ins and Outsof ActiveShieldingSubmitDocumentationFeedba ckCopyright 2015,TexasInstrumentsIncorporatedS1S2S3S 4 CAPACITANCETO DIGITALCONVERTERI2 CFDC1004 SDASCLVDDGNDOFFSET&GAINCALIBRATIONCONFIG URATION REGISTERS&DATA REGISTERSCAPDACMUXEXCITATIONMUXCIN1 CIN2 CIN3 CIN4 SHLD2 SHLD1 CHACHBS1S2S3S4 CAPACITANCETO DIGITALCONVERTERI2 CFDC1004 SDASCLVDDGNDOFFSET&GAINCALIBRATIONCONFIG URATION REGISTERS&DATA REGISTERSCAPDACMUXEXCITATIONMUXCIN1 CIN2 CIN3 CIN4 SHLD2 SHLD1 CHACHBT ypicalImplementationWiththe Shieldingin differentialmeasurements,SHLD1is assignedto CINnand SHLD2is assignedto CINm,wheren < m. For example,in the measurementCIN2 CIN1,whereCHA= CIN2and CHB= CIN1,SHLD1isassignedto CH1and SHLD2is assignedto this configuration,the CAPDACis Shieldingin DifferentialConfiguration,CAPDACD isabled6 CapacitiveSensing.

8 Ins and Outsof ActiveShieldingSNOA926A February2015 RevisedFebruary2015 SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporatedCIN1 TapeSHLD1 Plexiglas TableAirCIN1 TapeS1S1 SHLD1 Plexiglas TableAirCIN1 Plexiglas TableAirCIN1 TapeSHLD1 Plexiglas TableAirSHLD - LargerCIN OnlySHLD J Same SizeSHLD RingCIN1 SHLD1 Dielectric J cmCIN12 cmCIN1 CIN1 SHLD1 Underneath2 cm2 cmCIN1 OnlySHLD - LargerSHLD RingSHLD J Same Size2 cm2 cm2 cm2 cm2 experimentwith four differentshieldingconfigurationswas performedto determinewhatkind ofrelationshipshieldinghas with directivity,sensitivity,and sensortopologyemployedhereis mainlyusedfor proximityand gesturerecognitionapplications(groundedt argets).Thefour configurations,shownin Figure9 and Figure10, wereas follows:1. CIN1electrodeonly2. Shield1the samesize as CIN1and directlyunderneath3. Shield1larger(200%larger)thanCIN and directlyunderneath4. Shield1 Ringaddedon the sameplaneas CIN1with Shield1underneath(sameas configuration3).

9 Figure9. SensorLayouts- Top ViewFigure10. SensorLayouts- SideView7 SNOA926A February2015 RevisedFebruary2015 CapacitiveSensing:Ins and Outsof ActiveShieldingSubmitDocumentationFeedba ckCopyright 2015,TexasInstrumentsIncorporated showsthe side profileof the Plexiglas and tableare part of theexperimentaltest setupin Figure13 and had a thicknessof mm and 3 cm, plexiglasonly servedas a way to housethe sensorswith writtendistanceintervalsto actualsensorconfigurations3 and 4 prototypesare shownin Figure11 and Figure12 with the entiretest setupin ActualSensor- LargerSHLD1 UnderneathCIN1 Figure12. ActualSensor- SHLD1 RingaroundCIN1withSHLD1 UnderneathFigure13. ShieldingConfigurationTestSetup8 CapacitiveSensing:Ins and Outsof ActiveShieldingSNOA926A February2015 RevisedFebruary2015 SubmitDocumentationFeedbackCopyright 2015, explainedin the previoussections,shieldingthe sensorelectrodehelpsblockany externalinterferenceand experimentalresultsshowthat eventhoughshieldingdoesnot totallyeliminateall of theinterference,it doessignificantlyreduceit.

10 Figure14 displaysthe changein capacitanceversusthedistanceof the handto the sensorfromthe top side,consideredthe targetzonefor the caseis the mostcommondirectionof proximitydetectionwhereasthe proximityfromthe side (showninFigure15) is treatedas the unwantedinterference,requiringa the shieldsize increases,sodoesthe changein capacitancefromside is importantto notethat increasingthe areaof theshielddecreasessensitivityand dynamicrangeto someextentin the certainproximityrangeand marginfor interferenceand the shieldneedsto be sizedappropriatelyfor eachcase,sinceit doesnot havea linearrelationshipto rangeand : LiquidLevelApproachfor all datacollectedfromthis ExperimentalResultsof ProximityFromthe Top SideFigure15. ExperimentalResultsof ProximityFromthe SideThe capacitancemeasurementsof proximityfromthe bottomside showthat at a fixeddistanceawayfromthe sensor,thereis a of the interferencecannotbeeliminatedunlessthe shieldis muchlarger(an orderof magnitude)thanthe summary,Figure14 and Figure15 showthat eitherusinga shieldthat is the samesize as the sensor,or one that is 200%largerin area,haveaboutthe sameimpacton targetzonesensitivity,but usingalargershieldcan reducethe vulnerabilityto interferencefromthe February2015 RevisedFebruary2015 CapacitiveSensing.