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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 340). 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
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 115 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 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 88.
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Chapter 8 Tutorial
Measurement Fundamentals
371