- Use a separate measurement device such as another DMM or oscilloscope to verify the signal behaves as expected at the input terminals of the DMM. This isolates the DMM device as the error and ensures that the input signal is not being corrupted at some other point in your system.
- Self-calibrate your device using the niDMM Self Cal VI. See Self Calibration for more information.
Incorrect and Noisy Readings
- Use shielded cables instead of ribbon cables in noisy environments or when measuring mV signals in the absence of signal conditioning. Specific cabling recommendations for your device can be found in the NI-DMM Help.
- If performing a waveform acquisition, configure the sample rate to be at least 10 times your signal's frequency. Bad readings are often caused by aliasing, which is in turn caused by a sampling rate that is too slow. See Aliasing for more information.
For each data point returned by the NI-DMM driver, the A/D converter digitizes several samples. These samples are averaged to produce a more accurate result that is less susceptible to noise. For example, when taking a simple DC voltage measurement, the DMM takes 4 samples and averages them in hardware. The DMM driver then returns 1 data point. This filtering technique can be used to take more precise measurements of a noisy signal.
The aperture time
can be set to multiples of the power line frequency so that it rejects those unwanted frequencies. Typical power line frequencies are 60 Hz (16.67 ms) for the USA and 50 Hz (20 ms) for most other countries. For a 60 Hz system, an aperture time of 1 power line cycle (PLC) or 16.67 ms is the minimum to provide line rejection.
- Use the DC Noise Rejection Property so each DC reading the DMM returns is an average of multiple high-speed samples. By adjusting the relative weighting of those samples, you can adjust the sensitivity to different interfering frequencies. See DC Noise Rejection for more information.
- If your device has user-programmable input ranges, be sure that the input range selected is one that minimizes noise and optimizes accuracy for your signal range. You can find this information in the specification sheet for your module.
Returns and External Calibration
If you have exhausted all other troubleshooting tips and you still have offset or bad readings, you may need to send the board to NI for testing and re-calibration.
- Check the calibration interval for your device. If the device is not calibrated at the frequency of the recommended calibration interval, it is no longer guaranteed to meet the warranted specifications.
- Initiate a calibration request with NI.