This article explores the effect of clock tolerance on the performance of the lowpass decimation and digital filter inherent in sigma-delta ADCs, and especially the filter notch frequencies. Low-bandwidth sigma-delta applications commonly take advantage of the digital filter to provide 50Hz, 60Hz, or simultaneous 50Hz/60Hz noise rejection. It is important to understand the relationship between clock frequency and filter characteristics of the digital filter when choosing an external clock crystal or selecting an internal clock. ……