Design Of A Switched-capacitor Sigma-delta Modulator Based On Comparator

Sigma-Delta A/D converter is broadly used in audio digital signal processing systembecause of its high accuracy, high linearity, low cost and anti-aliasing to the sampled signal.The converter includes decimation filter and Sigma-Delta A/D modulator which is the coreelement.This paper designs an audio Sigma-delta modulator. With considering design cost andcomplexity, the modulator is realized by a second-order one bit single-loop structure. Matlabis used to establish the modulator system model in view of the factor which will affect thesystem performance. The designed Sigma-Delta modulator is realized by fully differentialswitched capacitor circuit which includes comparator-based integrator, quantizer, bandgapreference, non-overlapped clock. The sub-circuits are designed and simulated with Specture.In order to improve the accuracy of Sigma-Delta modulator, the first integrator uses apreset common-mode voltage control circuit. It can short the time of integral charge transferand lead to the integral current reduction,the integral precision improvement of the integrator,and decreasing of the output voltage error which is induced by the mismatch of currents.Thesimulation results show thattheintegrator common-mode output voltage error of the controlcircuit decreases by40%. The bidirectional dynamic current technology further enhances theintegrator precision, improves the CBSC integrator’s current source output resistance, andreduces the nonlinear error of the output voltage of the integrator.By adopting the CHRT18process to simulate under the condition of1.8V power supply,128oversampling rate, the circuit simulation indicates that the Sigma-Delta modulator canreach13bits resolution andother performance indicators can meet the design requirements.