A
G R E AT E R
M E A S U R E
O F
C O N F I D E N C E
Problem: Reading Drift
in Low Resistance
Measurements
Probable Cause:
Thermoelectric Voltages
Thermoelectric voltages are the most
common cause of reading drift in low resistance measurements. They are caused by
differences in temperature between different
parts of the measurement circuit.
Remedies
A. Current–Reversal Technique—Cancel
the thermal EMFs by making two measurements with currents of opposite polar-
ity. Apply the test current and measure
the voltage. Then reverse the polarity of
the test current and measure the voltage
again. Add the two voltage measurements and divide the result by two. This
completely cancels the thermal EMF in
the measurement.
This method requires a low noise voltmeter with a response speed faster than the
thermal time constant of the circuit. If the
response speed is too slow, any changes in
the circuit temperature during the measurement cycle will cause changes in the
thermal EMFs that will not be completely
cancelled, and some error will result.
B. Offset Compensation Technique—Make
two voltage measurements, one with the
test current enabled, the other with the
test current disabled. Subtract the results
of the second measurement (which consists of the thermal EMF) from the first
to get the true measurement.
Specifications are subject to change without notice.
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Problem: Reading Drift in Low Resistance Measurements
No. 2566
1004
October 2004
1