Effective dithering of sigma-delta modulators

Dithering methods for removing idle channel noise in sigma-delta (ΣΔ) modulators are presented. This noise exists for all types of ΣΔ modulators. Its structure appears as periodic impulses, the peak levels of which are much higher than their RMS values. The noise is readily apparent in the time domain, but not in the power spectrum. Relatively large dither power is required to remove this noise. Dithering with pseudorandom white noise is effective if the dither signal is shaped according to the quantization noise transfer function of the modulator. This allows the dither power to be relatively high without significantly increasing the noise in the baseband. The minimum dither power required for effectiveness is predominantly a function of the step size of the quantizer. Simulation and experimental results are presented