Why does my data acquisition system show more noise when measuring a signal than my DMM?
A digital voltmeter uses an integrating or averaging conversion method while most A/D boards takes a sample at a single point in time. The digital voltmeter averages a voltage reading during a certain time interval. Since most noise averages out over time, averaging makes the DMM very immune to high frequency noise. Most A/D boards pick off a voltage reading at the instant the A/D board was triggered. These two methods can return the same voltage reading or very different ones depending on the instant in time the A/D board was triggered, and the amount of noise present. Taking a large number of samples with the data acquisition board and performing a mathematical average on the data will usually provide data as noise free as that obtained from the DMM.
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