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This paper describes a new model for the stability analysis of low-pass Sigma-Delta modulators (Σ∆Ms) using the describing function method. The transfer of a single-bit quantizer is represented by a global signal gain and a phase uncertainty. This phase uncertainty arises from the limited accuracy in time with which the quantizer can detect the quantization-level crossings. This new model allows for a better prediction of possible idle patterns (important for the in-band signal-to-quantization error ratio), and calculation of zero-input stability boundaries for loop-filter parameters in higher order low-pass Σ∆Ms.

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