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You can use the 2-wire method to get accurate measurements above 100 kΩ. For lower resistance values, such as 100 Ω, the interconnecting cabling can add significant resistance that can greatly affect your measurement. Copper has a temperature coefficient in the 3,000 ppm/°C range, which can add instability to the measurement. NI recommends the Belden 83317E cable, which has excellent shielding and insulation resistance qualities and a resistance of 39 mΩ/ft. Refer to the Belden CDT Incorporated Web site at www.belden.com for information about this cable.
When considering cabling, refer to the following example:
Assume that a test system has 50 feet of copper interconnect cable hooked to a single 100 Ω resistance, such as a remote sensor device, the cable could easily introduce a resistance of ~2 Ω because Belden 83317E has a resistance of 40 mΩ/ft and 50 ft x 40 mΩ/ft = 2 Ω. The temperature coefficient of the resistance is as follows:
TC = 2Ω x 3,000 ppm/°C = 6 mΩ/°C
Relative to the 100 Ω resistance being measured:
TC = (6 mΩ/°C)/(100 Ω) = 60 ppm/°C
The error introduced by the copper not only affects the initial value of resistance measured but also introduces a temperature drift into the measurement. The drift with temperature of the copper resistance (60 ppm/°C in the example above) is much larger than the drift of the resistance ranges of the DMM (well under 10 ppm/°C). However, this drift might be perfectly acceptable to the particular measurement being made. Temperature drifts should be considered in the system error budget.
Sometimes the leads can be locally shorted, a measurement is made, and then this "offset" and its associated TC subtracted from the subsequent 2-wire resistance measurement on the resistors under test. This technique works with careful experimental measurement practice. An outline of the methodology for this technique in the context of an automated measurement system, with programmable switching available, is as follows:
This method is subject to the following caveats: