Chapter 8 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 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 HP 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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