E-2 Measurement Considerations Model 2750 Multimeter/Switch System User’s Manual
Measurement considerations
Low-level voltage measurements made using the Model 2750 can be adversely affected by
various types of noise or other unwanted signals that can make it very difficult to obtain
accurate voltage readings. Some of the phenomena that can cause unwanted noise include
thermoelectric effects (thermocouple action), source resistance noise, magnetic fields, and
radio frequency interference. The following paragraphs discuss the most important of
these effects and ways to minimize them.
NOTE For comprehensive information on low-level measurements, see the “Low Level
Measurements” handbook, which is available from Keithley.
Thermoelectric potentials
Thermoelectric potentials (thermal EMFs) are small electric potentials generated by dif-
ferences in temperature at the junction of dissimilar metals. The following paragraphs dis-
cuss how such thermals are generated and ways to minimize their effects.
Thermoelectric coefficients
As shown in Table E-1, the magnitude of thermal EMFs generated depends on the particu-
lar materials involved. Best results are obtained with clean copper-to-copper connections
as indicated in the table.
Table E-1
Material thermoelectric coefficients
Material Thermoelectric potential
Copper-Copper
Copper-Silver
Copper-Gold
Copper-Cadmium/Tin
Copper-Lead/Tin
Copper-Kovar
Copper-Silicon
Copper-Copper Oxide
0.2µV/°C
0.3µV/°C
0.3µV/°C
0.3µV/°C
1–3µV/°C
40µV/°C
400µV/°C
1000µV/°C
2750-900-01.book Page 2 Wednesday, August 3, 2011 7:56 AM
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