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.
Number of Averages
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.
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