A third-order multistage sigma-delta modulator with reduced sensitivity to nonidealities

A multistage third-order sigma-delta modulator, which is unconditionally stable and has a low sensitivity to component mismatch and op-amp performance limitations, has been designed and fabricated in a 1.2- mu m CMOS double-poly technology. The modulator, consisting of cascaded second- and first-order stages, is scaled to prevent performance degradation from integrator overload. In addition, the first-stage integrator output is used directly, instead of its quantization error, to facilitate ratioless input circuitry in the second stage. Experimental results indicate a signal-to-noise ratio of 93 and 90 dB at a signal-to-distortion ratio of 93 dB for sample rates of 24 and 80 kHz, respectively.>