I am trying to measure the DC voltage of a full-wave rectified 60Hz AC signal using an A/D and have it match what a Fluke meter reads on its DC scale. My current approach is to take as many (16 bit) samples as necessary in a measurement loop to measure a full 60 Hz cycle (16.6667 ms), then average the samples. I then take the A/D output and apply a scale factor to get the actual voltage reading to match what the Fluke says. But, when I apply a pure DC voltage (or a different rectified AC voltage) to the measurement circuit, the scaled result no longer matches the Fluke when measuring the same pure DC voltage. The results differ by more than 5% and the difference varies with peak-peak range of the rectified DC voltage.
So, I guess I'm asking how does a Fluke (or other) meter measure DC using it's internal A/D?