AN-256Circuitry for Inexpensive Relative Humidity …

1 ApplicationReportSNOA720B August1981 ,humidityis ,albeithighlyaccurate,arenotobviousandte ndtobeexpensiveandcomplex(SeeBox).Accura tehumiditymeasurementis vitaltoa numberofdiverseareas,includingfoodproces sing,paperandlumberproduction, , August1981 RevisedMay2013AN-256 CircuitryforInexpensiveRelativeHumidityM easurementSubmitDocumentationFeedbackCop yright 1981 2013, ,thesensordescribedbytheresponsecurve(Fi gure1) isinexpensiveandprovidesa furtherconsiderationin signalconditioningis ,it hasa C a chemicallytreatedstyrenecopolymerthathas a atitssurface,timeresponseis reasonablyrapidandis blockdiagramoftheconceptchosentoinstrume ntthesensorappearsin Figure2. Anamplitudestabilizedsquarewavethatis symmetricalaboutzerovoltsis usedtoprovidea precisionalternatingcurrentthroughthesen sor,satisfyingtherequirementfora fedintoa currentsensitive(forexample,theinputis atvirtualground)logarithmicamplifier, scaled,rectifiedandfilteredtoprovidea 40%is compensatedbybreakpointtechniquesin Figure3.

2 It is worthnotingthattheentirefunctiondescribe din Figure2requiresa accomplishedbynovelcircuitryapproaches,e speciallyin generatedbyA1,1 setupin a positivefeedbackconfiguration,causingit programmedbythe15 resistortocurrentlimitatabout5 resistorstringtostabilizeatabout 8 ' F capacitorreverses,causingtheamplifiertos witchagainwhenthecapacitorreachesa thresholdestablishedbythe120 divider(waveforms,Figure4). Thiscircuit'soutputis dependentuponthe+ C August1981 RevisedMay2013 SubmitDocumentationFeedbackCopyright 1981 2013, maybeusedin , C temperaturedependenceofthehumiditysensor is reducedbymorethananorderofmagnitudebythe LM334' thehumiditysensor' ,theLM334shouldbemountedin C temperaturecoefficientis negligiblysmallcomparedtothesensors 1% usedtodrivecurrentthroughthesensorandint othesummingjunctionofanother1/4ofA1,whic his connectedasa (Q1)in thefeedbackloopprovideslogarithmicrespon se,duetothewellknownrelationshipbetweenV BEandcollectorcurrentin ' expressedin alwaysatgroundpotentialthesensorseesther equiredsymmetricaldrive(waveforms,Figure 5).

3 Fedtoanother1 usedtosumin the40%RHtrimandprovideadjustablegaintose tthe100% filteredtoDCandroutedtoonehalfofA2,anLF3 53, August1981 RevisedMay2013AN-256 CircuitryforInexpensiveRelativeHumidityM easurementSubmitDocumentationFeedbackCop yright 1981 2013, usedtocompensatethesensordeparturefromlo garithmicconformitybelow40%RH(Figure1). Thisis accomplishedbychangingthegainoftheoutput amplifierforRHreadingsbelow40%.Theinputt otheoutputamplifieris 40%( attheoutputamplifiers + terminal) , offandthecircuitslinearizingfunctionis ,Q1'sDCoperatingpointwillvarywildlywitht emperatureandthecircuitnormallyrequiresc arefulattentiontotemperaturecompensation ,resultingin ,A3,anLM389audioamplifierICthatalsoconta insthreediscretetransistors,is usedin ,thecostofthelogfunctionis chiptemperaturesensorwhileQ2servesasa 1 k thistightlycontrolledthermalenvironment( typically50 C)

4 Andis regulatorensuressafeoperationoftheLM389, a 12V ,warm-upis 'sperformancefora stepfunctionof7 C changein shownin traceA whiletheLM389outputappearsin ,groundthebaseofQ2,applycircuitpowerandm easurethecollectorpotentialofQ3, ,calculatewhatQ3'scollectorpotentialwill beat50 C,allowing voltageclosetothecalculated50 C potentialattheLM389' fairlyloosetrim,astheexactchiptemperatur eis unimportantsolongasit ,ungroundQ2' 'scollectorvoltageandnotingstabilitywith in100 V ( C) ,placea 35k resistorin thesensorpositionandtrimthe150k ,substitutean8 M resistorforthesensorandtrimthe10kpotenti ometerforanoutputof4 ,substitutea 60M resistorforthesensorandselectthenominal4 0k valuein thebreakpointamplifierfora readingofRH= 24%.It valuetominimize hop atthecircuitoutputwhenthebreakpointis nowcalibratedandwillreadambientrelativeh umiditywhenthePCRC-55sensoris August1981 RevisedMay2013 SubmitDocumentationFeedbackCopyright 1981 2013, F anexpressiondenotingtheratioofwatervapor in , wettedthermometerbulbin a thepointatwhichwatersaturationoccursin C arerequiredtoproducethisphenomenonit is a August1981 RevisedMay2013AN-256 CircuitryforInexpensiveRelativeHumidityM easurementSubmitDocumentationFeedbackCop yright 1981 2013, HumiditySensors ;NewYork.

5 HumidityMeasurement InstrumentationTechnology; ;Watertown,Mass. HandbookofTransducersforElectronicMeasur ingSystems Norton,HarryN.;PrenticeHall,Inc.;1969. ElectricHygrometers Wexler,A.;NBSC ircular586;NBSW ashington, Anelegant6-ICcircuitgaugesrelativehumidi ty Williams,JamesM.;EDNM agazine, June5, August1981 RevisedMay2013 SubmitDocumentationFeedbackCopyright 1981 2013,TexasInstrumentsIncorporatedIMPORTA NTNOTICET exasInstrumentsIncorporatedanditssubsidi aries(TI)reservetherighttomakecorrection s,enhancements,improvementsandotherchang estoitssemiconductorproductsandservicesp erJESD46,latestissue,andtodiscontinueany productorserviceperJESD48, (alsoreferredtohereinas components ) aresoldsubjecttoTI s ,in accordancewiththewarrantyin TI s ,testingofallparametersofeachcomponentis productsandapplications, ,eitherexpressorimplied,is grantedunderanypatentright,copyright,mas kworkright,orotherintellectualpropertyri ghtrelatingtoanycombination,machine,orpr ocessin licensetousesuchproductsorservicesora licensefroma thirdpartyunderthepatentsorotherintellec tualpropertyofthethirdparty,ora TIdatabooksordatasheetsis permissibleonlyif reproductionis withoutalterationandis accompaniedbyallassociatedwarranties,con ditions,limitations, is solelyresponsibleforcompliancewithallleg al,regulatoryandsafety-relatedrequiremen tsconcerningitsproducts,andanyuseofTIcom ponentsin itsapplications.

6 Hasallthenecessaryexpertisetocreateandim plementsafeguardswhichanticipatedangerou sconsequencesoffailures,monitorfailuresa ndtheirconsequences, , ,TI s goalis , FDAC lassIII(orsimilarlife-criticalmedicalequ ipment)unlessauthorizedofficersofthepart ieshaveexecuteda enhancedplastic solelyattheBuyer's risk,andthatBuyeris solelyresponsibleforcompliancewithallleg alandregulatoryrequirementsin , , , :TexasInstruments,PostOfficeBox655303,Da llas,Texas75265 Copyright 2013,TexasInstrumentsIncorporat