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We can describe the process of choosing an A/D converter for an application as consisting of three steps: determining sampling rate, determining how to encode the analog voltage levels in a digital format, and determining the required resolution for the converter.
To determine an appropriate sampling rate, we must consider the Nyquist criterion, which states that a signal should be sampled at at least twice the highest frequency component contained in the signal.
The purpose of encoding is to establish a one-to-one correspondence between unique binary codes and analog voltage levels. To encode n voltage levels requires b = log n / log 2 bits. The resolution of the device is then (VRH-VRL)/n = (VRH-VRL)/2^b.