Specifications

Figure 4-6. Extended Ohms range.
4.3.7 Effects of Thermo-Voltaic Offset
Resistance measurements are sensitive to Thermo-Voltaic (Thermal EMF) errors, which can be caused by poor test
leads, relay contacts and other elements in the measurement path. These errors effect all measurement methods,
including 2-Wire, 4-Wire, 6-Wire and 3-Wire (guarded 2-Wire ohms). To quantify this error, consider a system in
which signals are routed to the DMM via a relay multiplexing system. Many vendors of switching products do not
provide Thermal EMF specification. It is common to find relays that have more than 50 µV EMF. With several
relay contacts in the path, the error is very significant. It is possible to measure it using the SMX2040 330mV DC
range. To do this, close a single relay that is not connected to any load, wait for a short time (about 2 minutes), than
measure the voltage across the shorted relay contacts. Make sure to short the DMM leads and set ‘relative’ to clear
the DMM offset prior to the measurement. To calculate worst case error, count all relay contacts which are in series
with the measurement (V,Ω+, V,Ω- terminals in 2-Wire, and I+, I- terminals in 4-Wire mode). To calculate the
worst case error, multiply this count by the Thermal EMF voltage. Use Ohms law to convert this voltage to
resistance error as in the following table.
Resistance Measurement Errors due to Thermo-Voltaic offsets.
SM2042
Range
Ohms
Current
DMM
Resolution
Error due to
10 µV EMF
Error due to
100 µV EMF
Error due to 1
mV EMF
33 Ω
10 mA
10 µΩ 1 mΩ 10 mΩ 100 mΩ
330 Ω
1 mA
100 µΩ 10 mΩ 100 mΩ 1 Ω
3.3 kΩ
1 mA
1 mΩ 10 mΩ 100 mΩ 1 Ω
33 kΩ
100 uA
10 mΩ 100 mΩ 1 Ω 10 Ω
330 kΩ
10 uA
100 mΩ 1 Ω 10 Ω 100 Ω
3.3 MΩ
1 uA
1 Ω 10 Ω 100 Ω 10 Ω
33 MΩ
100 nA
100 Ω 100 Ω 1 kΩ 100 Ω
330 MΩ
10 nA
10 kΩ 1 kΩ 10 kΩ 100 kΩ
31 Signametrics
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