High-order single-stage single-bit oversampling A/D converter stabilized with local feedback loops

A new method for the stabilization of high-order (>2) single-stage single-bit oversampling A/D converters is proposed. In this approach, the stability of the modulator is achieved by preventing any unbounded increase in the internal node-voltages through the insertion of local feedback signals inside the modulator loop. In the past, absolute bounds for stability have been derived for the first-order converter. This property is exploited in stabilizing a higher order loop by activating local first-order loops as soon as the internal integrators overload. With local feedback, individual integrators are prevented from saturating and the output voltages are within the proper bounds. The error caused by the local feedback signals is cancelled by feeding these signals through alternate signal paths, in a way similar to the quantization noise cancellation mechanism in a MASH architecture. Since the frequency of overloading can be made very low by proper design, the effect of imperfect cancellation due to mismatches in the two signal paths caused by the modulator nonidealities is quite small. Hence, compared to the inherently stable MASH architectures, the proposed approach achieves stability and is yet much less sensitive to component mismatches. In a sampled data environment where the integrator is realized using op amps, this translates into a low op amp gain requirement. Simulation results confirm that third order modulators using op amps with gain as low as 50 achieve a peak signal-to-noise ratio (SNR) of about 83 dB with an oversampling ratio of 64. This is less than 1 dB from the SNR achieved with infinite op amp gain