Using a separate measurement device such as another DMM (Digital Multimeter) or oscilloscope, verify that the signal is behaving as expected at the input terminals of your DMM device. 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.
Calibrate your device. Offsets, noisy and bad readings may also be caused by an analog to digital (A/D) converter that needs re-calibration. You may calibrate National Instruments data acquisition hardware manually by running a self-calibration in NI-MAX or using the VI niDMM Self Cal, although this is not effective in all cases and you may need to send your device to NI or a third-party lab for re-calibration. See Calibration for more information. Please consider the follow indications:
Recalibrates for time drift of onboard references
Corrects for AC flatness
Every two years
Recalibrates the measurement path and ADC for DC volts and resistance
Every 90 days or when the temperature changes more than 5 °C from last self-calibration
Recalibrates ADC drift for all ranges
Every reading when selected
Add to all specification 50 ppm of reading every 90 days and 3 ppm of reading/°C typical
Extended high-speed acquisitions
Offset errors may be caused by incorrect settings for your DMM hardware. To reduce the effect of unexpected offset, make sure to use ADC Calibration inserting niDMM Configure ADC Calibration and Auto Zero inserting niDMM Configure Auto Zero.
Also, you can perform Offset nulling measurements consists of measuring the corresponding zero reading in a measurement path and subtracting this value from subsequent samples.
Use shielded cables instead of ribbon cables in noisy environments, or when measuring mV signals in the absence of signal conditioning. Environmental noise may cause bad readings, especially if you have long wires (over 15 feet) between your transducers and your DMM board. See Field Wiring and Noise Considerations for Analog Signals for more information.
If you are performing a waveform acquisition, make sure to use a sample rate at least 10 times your signal's frequency, if possible. Many times, you may have to sample at even higher rates. 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.
You can also use the DC Noise Rejection Property, in this case each DC reading the DMM returns is actually an average of multiple high-speed samples. By adjusting the relative weighting of those samples, you can adjust the sensitivity to different interfering frequencies. Refer to DC Noise Rejection for more details.
If your device has user-programmable input ranges, be sure that the input range that is selected is that which minimizes noise and optimizes accuracy for your signal range. You can find this information in the specification sheet for your module.
If you have exhausted all other troubleshooting tips and you still have offset or bad readings, then you may need to send the board to National Instruments for testing and re-calibration. National Instrument's data acquisition products are shipped with a document that guarantees operation within the stated specifications for a certain period of time (typically one year) known as the calibration interval. If you have owned your hardware for longer than its calibration interval, then you may have inaccurate readings, however, before you may return a board, you must first contact a National Instruments technical support engineer, so that we may help you with your troubleshooting efforts. See National Instruments Hardware Repair or Calibration for more information.
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