For the ADCs discussed to this point, a time-varying signal was sampled or the ADC operated so rapidly that, for practical purposes, the signal did not change during a single conversion. Now we go to the other time extreme. What if there is a noisy signal, and we desire to digitize its average value? For example, what if we're looking at a flickering candle and we want to know the average illumination? One strategy would be to digitize at a high rate (say, with a successive approximations ADC) and average the results. But another strategy is to do signal averaging in the midst of the digitization process, thus simplifying the electronics. The dual slope ADC is one of several devices that work in this way.

How long does it take to go down a flight of stairs? "It depends how many steps there are," you obviously reply. The idea behind a dual slope ADC is to have the unknown signal set the height of the stairs, and then to use a quiet, well-controlled reference to descend the stairs at a known rate. If we know the rate, and we measure the time, we know how high the stairs are. Now let's translate that into electronics.