Masked Dithering of MASH Digital Delta-Sigma Modulators with Constant Inputs Using Linear Feedback Shift Registers

摘要

Digital delta-sigma modulators (DDSMs) are finite state machines; their spectra are characterized by strong periodic tones (so-called spurs) when they cycle repeatedly in time through a small number of states. This is particularly likely to happen when the input is constant or periodic. Dither generators based on linear feedback shift registers (LFSRs) are widely used to break up periodic cycles in DDSMs in order to eliminate spurs produced by underlying periodic behavior. Unfortunately, pseudorandom dither signals produced by LFSRs are themselves periodic and therefore may have limited effectiveness. This paper presents a rigorous mathematical analysis of DDSMs with LFSR-based pseudorandom dither, and a design methodology to achieve effectively masked dithering.