Digital noise-shaping of residues in dual-quantization sigma-delta modulators

A new dual-quantization Sigma-Delta modulator is proposed in this paper where the coarse-quantizer output, obtained from the fine-quantizer output by means of a digital noise-shaping coder, is fed back to the input of the first integrator by means of a p-bit digital-to-analog converter (DAC) (typically, p=1). To avoid the truncation error inserted into the first integrator to propagate to the rest of integrators, the residue of the digital coder is first integrated and then fed to the second integrator through an additional multibit DAC. Unlike other dual-quantization architectures, the proposed one allows to obtain a large signal-to-noise plus distortion ratio by using aggressive noise transfer functions, like in conventional multibit modulators. Mismatch effects on performances are carefully analyzed. It will be shown that more than one digital coder can be included in the architecture in order to reduce the number of bits of the additional DAC. Simulation results are presented which support the theoretical analysis.