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

The output signal of a digital delta-sigma modulator (DDSM) with a constant input may has a small fundamental period and therefore its spectrum may be characterized by a small number of strong periodic tones. Pseudorandom dither can be used to break up the tones, but it is indistinguishable from signal. The dither signal can be masked by using short sequence lengths but this may result in ineffective dithering and consequent tonal behavior. This paper shows how to use multiple shaped dither signals which are masked by the shaped quantization noise of the DDSM and yield a long fundamental output period.