Noise transfer functions and loop filters especially suited for noise-shaping SAR ADCs

Oversampling and noise-shaping have in recent years been introduced to SAR ADCs to improve the conversion accuracy. Similar to delta-sigma ADCs, this is done by means of a feedback loop containing a loop filter. In this paper, the high-level design of this loop filter is discussed, and important differences to classical delta-sigma loop filter design are pointed out. Among others, it is found that the poles of the noise transfer function, and not only the zeros, play a significant role on the conversion accuracy. Based on this, a new loop filter topology with four poles and two zeros is proposed and compared to existing loop filters. This reveals that the proposed loop filter can yield more energy-efficient noise-shaping SAR ADCs than the ones seen in the literature today.