This document provides recommended tips and techniques for making accurate resistance measurements with an NI Digital Multimeter (DMM). Use this document to make sure you are following best practices if your NI DMM is reading unexpected, inaccurate, or abnormal results.
The wire configuration you should use for your NI DMM depends on the target measurement. If the target measurement is more than 100 kΩ, use a 2-wire configuration. If the target measurement is less than 100 kΩ, use a 4-wire configuration.
The 2-wire method is commonly used as it is the simplest and most straightforward method. In 2-wire, you can get accurate measurements above 100 kΩ relatively easily.
For precision measurements with resistances below 100 kΩ, 4-wire works more reliably and conveniently than 2-wire. A 4-wire configuration requires 4-wire switching and more cabling; however, you may decide the tradeoff is acceptable, depending on the accuracy versus complexity requirements of your system.
Offset Compensated Ohms is a NI-DMM driver feature provided for eliminating thermal EMFs or offset voltages in a resistance test system. Offset Compensated Ohms applies to both 2-wire and 4-wire resistance.
Offset nulling measures the corresponding zero reading in a measurement path and subtracts the value from subsequent samples.
When measuring resistance in a system, it is important to consider the effects of cable resistance. It is also important to notice that interactions between scanning multiple resistances and system cabling can present time-dependent problems.
Learn more about system considerations for resistance measurements
Electrostatic noise pickup becomes a major concern when measuring high resistances. To prevent problems with noise pickup, proper shielding is critical when measuring resistances greater than 100 kΩ.
Learn more about optimizing for high-resistance measurements
A commonly overlooked source of noise is the source noise. The inherent noise generated by a non-ideal resistor is called Johnson noise. To get a meaningful measurement with resistances above 10 MΩ, you must have good shielding, including the resistor under test.
The Johnson noise of typical high MΩ resistors is listed below in ppm of reading, assuming a bandwidth of 10 Hz (roughly equivalent to a 100 ms aperture time).
Resistor Value R | Johnson Noise as ppm of reading (10 Hz bandwidth), p-p1 |
|---|---|
10 MΩ | 1 ppm |
30 MΩ | 5 ppm |
100 MΩ | 25 ppm |
300 MΩ | 140 ppm |
1 GΩ | 770 ppm |
1Represents the "ideal" noise on the best range for that resistance (ohms) measurement due to the Johnson noise of the resistor. Actual noise may be higher, due to environment, poor shielding, and so on.