Delta-Sigma Modulation Concepts

Summary

Delta-sigma modulation is a technique which combines filtering and over-sampling to perform analog-to-digital conversion: the noise from a low resolution quantizer is shaped away from the signal band prior to being removed by filtering. High-speed conversion can be accomplished by using a continuous-time filter inside the delta—sigma loop, and we are interested in this for its applicability to radio receiver and other high frequency circuits. Continuous-time modulators have other advantages such as built-in antialiasing and less switching noise. Performance of a ΔΣM is determined by taking the spectrum of a sequence of output bits generated from time-domain simulation of the modulator; it is characterized with some of the usual ADC performance measures such as DR and SNR, while omitting others which have no meaning in ΔΣsuch as DNL and INL. How to actually perform the time-domain simulation is a matter of considerable import in oversampled converters because they will usually require many more output samples than a Nyquist rate ADC before performance can be measured.