Noise considerations in high-accuracy A/D converters

The increased resolution of A/D converters makes it virtually impossible to follow the old design rules that dictated the noise of converter systems should be much less than the value of the least significant bit. The response of a successive-approximation MOS A/D converter system to noise is examined and an expression is derived for the distribution of output codes as a function of the noise. Because of the way a successive approximation register-type converter works, the distribution of codes is found to be non-Gaussian, with a nonmonotonic decay as one moves away from the principal mode of the distribution. The derived expression is found to be in excellent agreement with experimental results. The average value of the distribution of output codes is calculated and found not to be equal to the output code obtained from a noise-free converter. New designs with high-accuracy A/D converters must take into account the expected distribution of output codes rather than simply assuming that one will always get the `right' answer. Averaging over many conversion cycles will not remove all the effects of the noise.