XITRON XT2640 - Optimizing Low-Level DC Performance; Nternal DC Ero Djustment; External DC Zero Adjustment; Using an X Current Option Input at Low Levels

XT2640OperatingManual 13July2016 Page70of187
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13 OPTIMIZINGLOW‐LEVELDC PERFORMANCE
IfyouareonlyusingmeasurementsconfiguredasAC(i.e.notDCorAC+DC)ortheAClevelsaresignificantlyhigherthanany
expectedDClevels(e.g.morethan30:1higher)thenlowlevelDCperformanceisnotrequiredandthissectioncanbeignored.
13.1 INTERNALDCZEROADJUSTMENT
TheXT2640automaticallycheckstheinternalDCzeroesiftheenvironmenthassignificantlychangedandthenappliesanychanges
asneeded.TheautomatictrackingofenvironmentalchangescanbedisabledbyselectingOFFfortheAUTOZEROsettinginthe
PREFERENCESscreen,inwhichcasetheusershouldoccasionallyperformthisInternalDCZerooperationmanually.
ThisisachievedbypressingtheSYSCONFIGbuttonfromanymainscreenandthenpressingtheINTERNALbuttonintheDCZERO
area.TheXT2640willthenperformanInternalDCZerooperationandsavetheadjustmentsrecorded.Ascreenwillshowthe
progressoftheoperationandwillwaitforyoutopresstheRETURNbuttononthatscreenwhentheactionhasbeensuccessfully
completed.
Recommendations:
IfAUTOZEROhasbeensettoOFFthenitisrecommendedtoperformthisInternalDCZerooperationatleastdaily,or
whenevertheenvironmenthaschangedbymorethan±2Cfromthatinwhichitwaslastperformed.
IfAUTOZEROhasbeensettoONbuttheXT2640isinasignificantlydifferentenvironmentthatinwhichitwascalibrated
thentherewillbeafewminutesafterturningontheXT2640whiletheDCzeroesaretrackingthatchangeinenvironment.
ToreducethissmallshiftduringthefirstfewminutesofoperationyoumaywishtoperformanInternalDCZerooperation
inthenormallyusedenvironment,otherwisethereisnoneedtoperformthisoperation.
Note:
ThisoperationtakesafewsecondsanddoesnotrequirethatsignalsberemovedfromtheXT2640terminals.Notethat
thisoperationonlyaffectschannelswhichareconfiguredforuse.
WhileperformingtheInternalDCZeroyoushouldnotsendanyconfigurationinterfacecommandstotheXT2640.
13.2 EXTERNALDCZEROADJUSTMENT
IfyouareusingexternalcurrenttransducersorshuntswhichmayhaveasignificantDCoffsetintheiroutputyoucanperforman
ExternalDCZerooperationtosetthoseoffsetstothepresentlymeasuredvalues.ThisexternalDCzerooperationcanalsobeusedto
accountforanyexternalDCoffsets(leakagecurrentsorthermallyinducedvoltages)ortoadjustanyremainingoffsetsintheXT2640
causedbyadverseenvironments(e.g.nearbyheatsourcesand/orunusualorientationoftheXT2640).
ThisdiffersfromtheInternalDCZerodescribedabovebecauseitcorrectsforanyexternallyappliedDCoffset,soyoumustensure
thattherearenoDCsignalspresentontheterminalsoftheXT2640whicharenotwishedtobeadjustedfor.
ThisisachievedbypressingtheSYSCONFIGbuttonfromanymainscreenandthenpressingtheEXTERNALbuttonintheDCZERO
area.YouarethenshownascreenwhichrequeststhatyouselectwhethertoadjusttheDCoffsetforallchannels(ALL),oronly
thoseconfiguredforscaling(ONLYSCALED),ortoaborttheaction(NO).AfteryouselectthedesiredchoicetheXT2640will
immediatelyusethelatestobtainedDCmeasurementsandapplythemasDCoffsets.Ifnochannelswereaffectedbythisoperation
thenamessageisdisplayedforashorttime.IfthemeasuredDCvaluesweretoolargetobeadjustedthenamessageisdisplayedfor
ashorttimeandnoadjustmentsaremade.
Recommendation:
IfitisdesiredtousethisexternalDCzeroabilitywhenAUTOZEROissettoOFFthenitisrecommendedtofirstperforman
internalDCzerooperationandensurethatvalidmeasurementresultsareavailablebyreturningtoanyofthe
measurementresultsscreensandcheckingthatthereadingsbeingindicatedarethoseexpectedbeforereturningtotheSYS
CONFIGscreenandperformingtheexternalDCzerooperation.
Note:
Thistakesverylittletimeandhasanimmediateeffect.Channelswhicharenotconfiguredarenotaffectedbythisaction.
13.3 USINGANXCURRENTOPTIONINPUTATLOWLEVELS
Xoptioncurrentinputshavearesolutionof1µVorlowerwhenusedwithanexternalcurrentshunt.Toachievethislevelof
performancetheusermustbeawareofthermallyinducedDCvoltageswhichcanoccurbothwithintheshuntitselfandinthewiring
betweentheshuntandtheterminalsoftheXT2640.Theusercanreducetheseeffectsbyusinghighqualitywiringandconnectors,
andbalancingthetwosensewiresbyusingthesamegagesandlengths,usingthesameconnectortypes,androutingthetwowires
together.Inseverecircumstancesthiscanalsobeaffectedbyaircurrentspassingovertheshuntorthewiring,soyoumayneedto
takeprecautionstopreventunwantedairflowoverthem.
Recommendations:
TwistingtogetherthetwosensewiresfromtheshunttotheXT2640terminalsbothreducesthesethermalvoltageaffects
andalsoreducestheinductivepick‐upofACcurrentinthesewires,soishighlyrecommended.Youshouldalsonotethat
theuseofthinnergagewiring(e.g.26awgorthinner)willproducethebestthermalvoltageperformanceasthinnerwiring
doesnotallowsignificantheattransferthroughit.
Theuseofcoaxialcableisnotrecommendedasoftenthetwoconductorsarenotofthesamematerial.
Evenwhenallprecautionsaretaken,therewilltypicallybeseveralmicro‐voltsofthermallyinducedoffsetvoltages.The
regularuseoftheExternalDCZerocapabilitydescribedaboveishighlyrecommendedwhenusingshunts.