Digitally-enhanced high-order ΔΣ modulators

The input feedforward path in a DeltaSigma modulator is an attractive technique for low-distortion swing-reduction design. It helps lower the power dissipation, especially in DeltaSigma modulators designed with low oversampling ratios (OSRs) in low-voltage nanometer CMOS technologies. However, a DeltaSigma modulator with analog feedforward (AFF) requires an analog adder before the quantizer, which can limit the achievable resolution or degrade the signal swing and increase the power dissipation. In this paper, a single-stage multibit DeltaSigma modulator with digital feedforward (DFF) is proposed to realize a high-order finite-impulse-response noise transfer function, thereby achieving high signal-to-quantization-noise ratios at low OSRs. Its key features include reduced swing at the opamp outputs, reduced sensitivity to integrator nonlinearities, and robustness to DeltaSigma modulator coefficient variations, all of which are achieved using only minimal additional digital hardware. Behavioral simulation results confirm that the proposed DFF modulator achieves the swing-reduction and low-distortion performance of an AFF modulator, while eliminating the need for an analog adder.