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 si ngle-bit oversampling A/D converters is proposed. In this approach, th e stability of the modulator is achieved by preventing any unbounded i ncrease in the internal node-voltages through the insertion of local f eedback signals inside the modulator loop. In the past, absolute bound s 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 satura ting and the output voltages are within the proper bounds. The error c aused by the local feedback signals is cancelled by feeding these sign als through alternate signal paths, in a way similar to the quantizati on noise cancellation mechanism in a MASH architecture. Since the freq uency of overloading can be made very low by proper design, the effect of imperfect cancellation due to mismatches in the two signal paths c aused by the modulator nonidealities is quite small. Hence, compared t o the inherently stable MASH architectures, the proposed approach achi eves stability and is yet much less sensitive to component mismatches. In a sampled data environment where the integrator is realized using opamps, this translates into a low opamp gain requirement. Simulation results confirm that third order modulators using opamps with gain as low as 50 achieve a peak signal-to-noise ratio (SNR) of about 83 dB wi th an oversampling ratio of 64. This is less than 1 dB from the SNR ac hieved with infinite opamp gain. In this modern day of low voltage CMO S design, such a low opamp gain can be easily realized since no cascod e stage is required.