Quantization error spectrum of sigma-delta modulators

It is shown that the spectra of the quantization noise of first- and second-order sigma-delta modulators contain strong peaks that depend on the amplitude of the input signal and are intolerable in many applications. This is contrary to the assumption that the quantizer noise from sigma-delta modulators is approximately white. An alternative modulator architecture comprising a cascade of first-order sigma-delta modulators is shown to have a white error spectrum. It is concluded that cascaded sigma-delta modulators exhibit a smooth noise spectrum and are thus suitable for direct replacement of Nyquist-rate PCM converters.<>