Capacitive Proximity Sensing Using the FDC1004 …

1 ApplicationReportSNOA928A March2015 RevisedApril2015 CapacitiveProximitySensingUsingthe FDC1004 DavidWangABSTRACTC apacitiveproximitysensingcan be implementedin a widevarietyof applicationswith the use of TI'sFDC1004and with the flexibilityof the sensordesignin onlyare thereadvantagesof usingcapacitiveproximitysensingcomparedt o alternativedetectionmethods,therearealso guidelinesto followto ensuremaximumperformanceand stabilitywiththe detailthe basicsof proximitysensingand sensortopologyconsiderationsthat of Figures1 FringingElectricFieldsof the IsolatedSensorand the ElectricFlux Densityfor the DistanceComparisonfor Groundedand Distancefor of Tables1 Comparisonof ,HumanHand(GroundedTarget).

2 64 ProximitySensingRangeBasedon trademarksare the propertyof March2015 RevisedApril2015 CapacitiveProximitySensingUsingthe FDC1004 SubmitDocumentationFeedbackCopyright 2015,TexasInstrumentsIncorporatedSHIELDG NDSENSORSENSORSHIELDG roundedGroundedIsolated SensorParallel FingersBasicsof ProximitySensingUnlikea parallelplatetopologythat workson the principlesof the parallelplatecapacitor,the topologiesfor proximitysensinguse the fringingelectricfieldsto measurethe capacitance,as shownin Figure1. Themajorityof proximitysensingapplicationsuseseitherth e parallelfingersor the the bottomsidesof the electrodesis commonin mostapplicationsto reducethe noiseand strayparasiticcapacitancesin the surroundingenvironmentfromaffectingthe moreinformationon shieldingin capacitivesensing,referto theCapacitiveSensing:Ins and Outsof ActiveShieldingapplicationnote(SNOA926).

3 Figure1. FringingElectricFieldsof the IsolatedSensorand ParallelFingersTopologyModelingthe fringingeffectand workingthroughthe calculationsrequiresthe use of a showsa FiniteElementMethodsMagnetics(FEMM)simul ationof the electricflux densityof theparallelfingerstopologywith a purpleregionsin Figure2 representthe highestdensityof the densityand intensityof the electricfieldsare highestin the regionclosestto the inneredgesof the sensorand groundelectrodes,and will exhibitthe ,a humanfinger(groundedtarget)

4 In betweenthe electrodeswouldcontributemoretowardsthem easuredcapacitancecomparedto the fingernearthe outeredgesof eitherof the FEMMS imulationof the ElectricFluxDensityfor the ParallelFingersTopology2 CapacitiveProximitySensingUsingthe FDC1004 SNOA928A March2015 RevisedApril2015 SubmitDocumentationFeedbackCopyright 2015, of the mostcommontopologiesfor proximitysensingare the parallelfingersand the theirown advantagesand disadvantagesbasedon the type of targetthat is showsa summaryof the best sensortopologyfor differentsystemobjectivesand Comparisonof SensorTopologiesSystemObjectiveTopologyS electionWhySensinggroundedobjectssuchas the humanIsolatedsensorallowsthe majorityof field linestoIsolatedsensorbodyterminateto the humanbodywithoutGNDelectrodeDielectrican d capacitancechangeis

5 SmallcomparedtoSensingungroundedobjectsE itherthresholddetectionNo dedicatedGNDelectrodenearbyto terminatefieldHigherproximitydistancesen sitivityIsolatedsensorlinesNo dedicatedGNDelectrodenearbyto terminatefieldHighersensitivityand dynamicrangeIsolatedsensorlinesLessrisk for saturatedmeasurementsParallelfingersIsol atedgroundplaneThe isolatedsensoris the best topologyfor variousmaterials,sensingrangedetection,a nd sensitivitybutthe parallelfingerstopologyis capableof performingbetterin primaryconsiderationin selectingthe sensortopologyis how the electrodesare coupledto a FDC1004 Capacitive -to-DigitalConvertercan accommodatea capacitiveoffsetof up100 pF.

6 If the groundplane/electrodeis coupledtightlyso that the capacitancemeasuredbetweenthesensorelect rodeand groundis largerthanthe 100-pFmaximumoffsetcapabilitiesof the FDC1004 ,thecapacitancemeasurementswill alwaysbe takenin the size of electrodesandPCBstackupof the sensordesignto avoidsaturationdue to a parallelfingersdesigncan be less susceptibleto saturationif an isolatedgroundplaneis presentin the systembecausethe GNDelectrodeand groundplanewill not be countlesstargetmaterialsthat can be sensedusingthe capacitiveapproach,but it is possibletogroupthesetargetsinto two categories.

7 Groundedtargetsand ungroundedtargetscategories,low and high dielectricconstantmaterialscan be distinguishedin showsthe dielectricconstantsof materialsthathavelow dielectricconstants(closeto the dielectricconstantof air) are limitedto very smallsensingrangessincethe changein capacitance,dictatedby the parallelplatecapacitorequation,is small. Groundedtargetexamples humanbody,metalplates/cases Ungroundedtargetexamples Low dielectricconstant air, plastic,plexiglass,wood Highdielectricconstant varioustypesof alcohol,waterTable2.

8 DielectricConstantsof CommonMaterialsMaterialDielectricConstan tAir1 Alcohol16 123 SNOA928A March2015 RevisedApril2015 CapacitiveProximitySensingUsingthe FDC1004 SubmitDocumentationFeedbackCopyright 2015, DielectricConstantsof CommonMaterials(continued)MaterialDielec tricConstantWood2 6 Waterat 20 50 TopologyAnalysisBothtopologieshavethe capabilityto maindifferencebetweenthetopologieswith detectinggroundedobjectsis the sensitivityof the the locationofthe nearestcommongroundpotentialsource,the isolatedsensortopologyis typicallymoresensitivethanthe parallelfingerstopologyespeciallyat longerdistancesawayfromthe ungroundedtargets,the two topologieshavesimilarperformance.

9 But measurementscan saturatefor the isolatedsensorif the couplingto a groundplaneor groundsourceis largerthanthe 100-pFoffsetrangeof showsplotsof capacitanceversusrangefor groundedand groundedtargets,bothtopologiesshowgoodse nsitivity,with the isolatedsensorshowingslightlybetterperfo rmanceversusthe ungroundedtargets,bothtopologiessufferad ecreasein sensitivity,showinglowerdynamicrangeand interceptionof the noisefloorat FEMM simulationdatashownin Figure3 doesnot factorin othergroundsourcesin simulationdataonly takesinto considerationthe targetobjectand the electrodes.

10 Sensor,ground,and FEMM simulationsuseda sensorsize of 4 cm 1 cm for the isolatedsensorwith a shieldlayerof thesamesize 1 mm parallelfingerstopologywas pairedwith a GNDelectrodethe samesize with a 5-mmgap spacingbetweenthe sensorand shieldlayerspannedfromtheouteredgesof the two electrodes1 mm Capacitancevs DistanceComparisonfor Groundedand UngroundedTargetsOne advantagethat the parallelfingerstopologyhas overthe isolatedsensorwith ungroundedtargetsissensitivitybasedon locationof the the groundsourceis