MMR SB1000 - Page 10

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GeneralOperation
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However,oneshoulduseasmalltemperaturedeviationΔTinordertoobtainrepresentativedata
inthetemperaturedomain.Therefore,thevaluesofV1andV2willalsobesmall.Becauseofthis,
a direct measurement will not give high accuracy because of instrumental errors, and any
undesired
thermo‐voltageeffects fromwires,connectors,etc.Theseeffectscancreatesubstantial
offsetvoltageswhich,inadditiontothetemperaturedriftsandoffsetsoftheinputamplifiers,can
contributeamajorsourceofmeasurementerror.Thesecanbeeliminated,however,bytaking
measurementsattwodifferenttemperatureoffsets,usingtwodifferent
powersettings,andthen
usingthedifferencesignal.Weshowthisasfollows;therealvaluesofV1andV2acquire dbythe
SB1000aregivenby:
 (4).V1(P
1)=ε1 ∆T(P1)+ ∆V1and
 (5).V2(P
2)=ε2∆T(P2)+ ∆V2
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WhereΔV
1andΔV2aretheinstrumentandextraneousthermaloffsetvoltagesdiscussedabove,
andP
1isthepowerapplie d totheheateratthefirstmeasurementpoint.Now,iftheheaterpower
ischangedtoanewvalueP
2,weobtainasecondpairofvaluesforV1andV2:
 (6).V1(P
2)=ε1 ∆T(P2)+ ∆V1and
 (7).V2(P
2)=ε2∆T(P2)+ ∆V2
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TheoffsetvoltagesΔV1andΔV2canbeassumedtobeindependentofpowerP,becauseonlythe
temperature in the immediate neighborhood of the reference and sample junctions change, not
thosewheretheseotheroffsetvoltagesoriginate.
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Subtractingequations(6)and(7)from(4)and(5),
respectively,weobtainthetruevalueofε1from
thefollowing:
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 (8).
ε1=ε2{V1(P1)‐V1(P2)}/{(V2(P1)‐V2( P2)}
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(8)doesnotincludeΔV1andΔV2.Theoffsetshavebeenremoved.
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Certainhardwareandsoftwareprecautionshavealsobeenimple mentedtoeliminateanypossible
electricalcouplingbetweentheheaterandmeasurementcircuitsas well. 
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