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Tip 2: Measuring an AC Signal with a DC Offset
An ac-coupled true RMS measurement is desirable in situations where you are measuring small ac signals in the presence of large dc offsets. For example, this situation is common when measuring ac ripple present on dc power supplies. There are situations, however, where you might want to know the ac+dc true RMS value. You can determine this value through a manual calculation, shown below, add the square of the dc measurement and the square of ac measurement and take the square root. You should perform the dc measurement using at least 10 power line cycles of integration to reject the ac signal.
True RMS ac + dc =SQRT( ac^2 + dc^2)
Even if your multimeter contains a built-in function to measure the True RMS ac+dc value. You may want to manually determine the value using a dc measurement, an ac only measurement and apply the formula. The manual method will yield a better result, particularly when the dc and ac values are of different magnitudes.