Reducing Noise with DMM Measurements
Hardware: Modular Instruments>>Digital Multimeters (DMM)
Problem: How can I reduce noise with my DMM measurement?
Solution: Noise can be produced from a variety of factors, including: long cable length, bad connections, electromagnetic fields from other devices, etc. Thus, noise can be reduced in a few ways:
- Eliminate factors causing noise.
For more information on this please refer to the document: Field Wiring and Noise Considerations for Analog Signals.
- Configure the DMM measurement to average more samples per data point.
For each data point returned by the DMM driver, the analog-to-digital converter actually 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 actually 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.
Measurements can be made even more accurate by increasing the number of samples to average. By averaging more data points, it is possible to improve the measurement’s precision. In LabVIEW use the niDMM property node to change the property “Number of Averages”. In C, the property "NIDMM_ATTR_NUMBER_OF_AVERAGES" can be changed by calling the function niDMM_SetAttributeViInt32.
- Set the aperture time to be a multiple of the power line frequency.
Aperture time is the period during which the ADC is reading the input signal. By increasing the aperture time, we can potentially improve the accuracy of our measurement because we are measuring for a longer time and internally averaging the samples together. The aperture time can also 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. So for a 60 Hz system, an aperture time of 1 powerline cycle (PLC) or 16.67 ms is the minimum to provide line rejection. The aperture time can also be set with a property node.
Related Links: Developer Zone Tutorial: Understanding the FlexDMM Architecture Developer Zone Tutorial: Field Wiring and Noise Considerations for Analog Signals
Product Page: Digital Multimeters
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Report Date: 02/03/2006
Last Updated: 07/27/2007
Document ID: 3U2CUT2W
