Work is presented that shows that the behaviour of idle tones in Sigma-Delta modulators depends on whether the noise transfer function has zeros at dc or not. Simulation results for a variety of transfer functions of the same order but a different shape are presented. In particular, no zeros at dc, equiripple zeros, and all zeros at dc. It shows that, without dither, the effects of different filter transfer function shapes on idle tones is minimal for a given order. However, when dither is applied noise transfer functions with all the zeros at dc, Butterworth shapes, are better because lower levels of dither are required to eliminate idle tones.
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