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Use the following guidelines when determining a strategy for improving DC and RMS measurements:

• If a signal is overlapped with a single tone, longer integration times increase the accuracy of the measurement. If you know the exact frequency of the sine tone, use a measurement time that corresponds to an exact number of sine periods. If you do not know the frequency of the sine tone, apply a window, such as a Hanning window, to reduce significantly the measurement time needed to achieve a specific accuracy.
• If a signal is overlapped with many independent tones, increasing measurement time increases the accuracy of the measurement. As in the single tone case, using a window significantly reduces the measurement time needed to achieve a specific accuracy.
• If a signal is overlapped with noise, do not use a window. In this case, you can increase the accuracy of the measurement by increasing the integration time or by preprocessing or conditioning the noisy signal with a lowpass (or bandstop) filter.

RMS Levels of Specific Tones

You always can improve the accuracy of an RMS measurement by choosing a specific measurement time to contain an integer number of cycles of sine tones or by using a window function. The measurement of the RMS value is based only on the time domain knowledge of the signal. You can use advanced techniques when you are interested in a specific frequency or narrow frequency range.

You can use bandpass or bandstop filtering before RMS computations to measure the RMS power in a specific band of frequencies. You also can use the fast Fourier transform (FFT) to pick out specific frequencies for RMS processing.

The RMS level of a specific sine tone that is part of a complex or noisy signal can be extracted very accurately using frequency domain processing, leveraging the power of the FFT, and using the benefits of windowing.