C-4
Measurement Considerations
Minimizing thermal EMFs
To minimize thermal EMFs, use only copper wires, lugs, and test leads for the entire test
setup. Also, it is imperative that all connecting surfaces are kept clean and free of oxides. As
noted in
, copper-to-copper oxide junctions can result in thermal EMFs as high as
1mV/°C.
Even when low-thermal cables and connections are used, thermal EMFs can still be a
problem in some cases. It is especially important to keep the two materials forming the junction
at the same temperature. Keeping the two junctions close together is one way to minimize such
thermal problems. Also, keep all junctions away from air currents; in some cases, it may be
necessary to thermally insulate sensitive junctions to minimize temperature variations. When a
Cu–Cu connection is made, sufficient pressure must be applied to ensure the connection is gas
tight to prevent future oxidation.
In some cases, connecting the two thermal junctions together with good thermal contact to a
common heat sink may be required. Unfortunately, most good electrical insulators are poor
conductors of heat. In cases where such low thermal conductivity may be a problem, special
insulators that combine high electrical insulating properties with high thermal conductivity may
be used. Some examples of these materials include hard anodized aluminum, sapphire, and
diamond.
Nulling residual thermal offsets
Even if all reasonable precautions are taken, some residual thermal offsets may still be
present. These offsets can be minimized by using the Model 2182 Relative feature to null them
out. To do so, place the instrument on the 3mV range and short the end of the connecting cable
nearest the measured source (first disconnect the cable from the source to avoid shorting out the
source). After allowing the reading to settle, press the front panel REL button to null the offset.
Select the appropriate range, and make your measurement as usual.
If the offset voltage varies, the DC current-reversal technique should be used instead of REL.
The DC current-reversal technique requires a source that can output currents equal in magnitude
but opposite in polarity. In general, a voltage measurement is performed on both the positive and
negative alternations of the current source. The averaged difference of those two readings
cancels the thermal EMF component of the measurements. The Model 2182 can automatically
perform the measurements and calculate and display the result by using the Delta measurement
mode. See “
Source resistance noise
Noise present in the source resistance is often the limiting factor in the ultimate resolution
and accuracy of Model 2182 measurements. The paragraphs below discuss the generation of
Johnson noise as well as ways to minimize such noise.
Summary of Contents for 2182
Page 1: ...www tek com keithley Model 2182 2182A Nanovoltmeter User s Manual 2182A 900 01 Rev B May 2017...
Page 18: ......
Page 22: ......
Page 23: ...1 Getting Started Getting Started...
Page 41: ...2 VoltageandTemperature Measurements Voltageand Temperature Measurements...
Page 68: ...2 28 Voltage and Temperature Measurements...
Page 69: ...3 Range Digits Rate andFilter Range Digits Rate andFilter...
Page 82: ...3 14 Range Digits Rate and Filter...
Page 83: ...4 Relative mX b and Percent Relative mX b andPercent...
Page 91: ...5 RatioandDelta Ratioand Delta...
Page 117: ...6 Buffer Buffer...
Page 123: ...7 Triggering Triggering...
Page 140: ...7 18 Triggering...
Page 141: ...8 Limits Limits...
Page 149: ...9 SteppingandScanning Steppingand Scanning...
Page 168: ...9 20 Stepping and Scanning...
Page 169: ...10 AnalogOutput Analog Output...
Page 175: ...11 RemoteOperation Remote Operation...
Page 205: ...12 CommonCommands Common Commands...
Page 221: ...13 SCPISignalOriented Measurement Commands SCPISignalOri entedMeasure ment Commands...
Page 225: ...14 SCPIReferenceTables SCPIRefer enceTables...
Page 239: ...15 AdditionalSCPI Commands Additional SCPICom mands...
Page 260: ...15 22 Additional SCPI Commands...
Page 261: ...A Specifications Specifications...
Page 263: ...B Statusand ErrorMessages StatusandError Messages...
Page 268: ...B 6 Status and Error Messages...
Page 269: ...C Measurement Considerations Measurement Consider ations...
Page 278: ...C 10 Measurement Considerations...
Page 279: ...D Model182Emulation Commands Model182 EmulationCom mands...
Page 284: ...D 6 Model 182 Emulation Commands...
Page 285: ...E Example Programs ExamplePro grams...
Page 293: ...F IEEE 488 BusOverview IEEE 488Bus Overview...
Page 307: ...G IEEE 488andSCPI ConformanceInformation IEEE 488and SCPIConform anceInforma tion...
Page 310: ...G 4 IEEE 488 and SCPI Conformance Information...
Page 311: ...H Measurement Queries Measurement Queries...
Page 316: ...H 6 Measurement Queries...
Page 317: ...I Delta PulseDelta and DifferentialConductance Delta Pulse Deltaand Dif ferentialCon ductance...