Digitally-enhanced 2nd-order ΔΣ modulator with unity-gain signal transfer function

Low distortion and reduced swing can be achieved by designing the DeltaSigma modulator with an input feedforward path. Furthermore, the modulator stability can be enhanced and the requirements on the preceding anti-aliasing filter can be relaxed by designing the DeltaSigma modulator for a flat unity-gain signal transfer function (STF). Previously-developed DeltaSigma modulators with analog feedforward (AFF) achieved a unity-gain STF. However, their performance is limited by the need for an analog adder before the quantizer. Previously-developed DeltaSigma modulators with digital feedforward (DFF) eliminated the need for this analog adder. However, their STF was not unity. In this paper, a technique for designing digitally-enhanced DeltaSigma modulators with both a DFF path and a unity-gain STF is presented. A 2nd-order DFF DeltaSigma modulator with a unity-gain STF is then proposed. Its key features include reduced signal swing at the opamp outputs, reduced sensitivity to integrator nonlinearities, and robustness to DeltaSigma modulator coefficient variations. Behavioral simulation results confirm its performance advantages versus previously-developed AFF and DFF DeltaSigma modulators.