Agilent Technologies 34972A User Manual page 293

Chapter 7 Tutorial
Measurement Fundamentals
Offset Compensation Most connections in a system use materials that
produce small DC voltages due to dissimilar metal-to-metal contact
(thermocouple effect) or electrochemical batteries (for a description of the
thermocouple effect, see page 262). These DC voltages also add errors to
resistance measurements. The offset-compensated measurement is
designed to allow resistance measurements in the presence of small DC
voltages.
Offset compensation makes two measurements on the circuit connected
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to the input channel. The first measurement is a conventional resistance
measurement. The second is the same except the internal DMM's test
current source is turned off (essentially a normal DC voltage
measurement). The second measurement is subtracted from the first
prior to scaling the result, thus giving a more accurate resistance
measurement. Refer to "Offset Compensation" on page 132 for more
information.
Offset compensation can be used for 2-wire or 4-wire ohms
measurements (but not for RTD or thermistor measurements). The
34970A/34972A disables offset compensation when the measurement
function is changed or after a Factory Reset (*RST command). An
Instrument Preset (SYSTem:PRESet command) or Card Reset
(SYSTem:CPON command) does not change the setting.
If the resistor being measured does not respond quickly to changes in
current, offset compensation will not produce an accurate measurement.
Resistors with very large inductances or resistors with large parallel
capacitance would fall into this category. In these cases, the channel
delay parameter can be increased to allow more settling time after the
current source is switched on or off, or offset compensation can be turned
off. For more information on channel delay, see page 105.
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