An efficient technique to eliminate quantisation noise folding in double-sampling ΣΔ modulators

ΣΔ-modulation is a proven method to realize high-resolution A/D converters. A particularly efficient way to implement such a modulator uses double-sampling where the sampling frequency is twice the master-clock frequency. Unfortunately path mismatch between both sampling branches causes a part of the quantisation noise to fold from the Nyquist frequency back in the signal band. This degrades the performance. In this paper we show that multi-bit quantisation provides a partial solution for this problem. Next we present a true solution. The approach consists of modifying the quantisation noise transfer function of the modulator to have one or several zeros at the Nyquist frequency. This way the effect of noise folding can nearly be eliminated. It is shown that this can be implemented by a simple modification of one of the integrators of the overall modulator circuit. Finally several design examples of single-bit and multi-bit modulators are discussed.