Second-Order ΔΣAD Modulator with Novel Feedforward Architecture

This paper proposes novel feedforward architecture of a second-order ΔΣAD modulator with single DACfeedback topology. ΔΣAD modulator realizes high resolution by oversampling and noise shaping technique. However, its SNDR (Signal to Noise and Distortion Ratio) is limited by the dynamic range of the input signal and non-idealities of building blocks, particularly by the harmonic distortion in amplifier circuits. Compared with a feedbacked ΔΣAD modulator, in a full feedforward ΔΣAD modulator structure, the signal transfer function is unity under ideal circumstances. It means that the signal swings through the loop filter become smaller. Therefore, the harmonic distortion generated inside the loop filter can be significantly reduced because the effect of non-idealities in amplifiers can be suppressed when signal swing is small. Moreover, the reduction of the internal signal swings also relaxes output swing requirement for amplifiers in low-voltage design. However, in conventional feedforward ΔΣAD modulator, an analog adder is needed before quantizer. Especially in a multibit modulator, an additional amplifier is necessary to realize the summation of feedforward signals, which leads to large chip area and extra power dissipation. In this paper, we propose a novel architecture of a feedforward ΔΣAD modulator. It realizes the summation of feedforward signals without additional amplifier that is equivalent to the conventional one but smaller chip area and low-power dissipation. We also conducted MATLAB and SPICE simulations to verify the proposed architecture and modulator circuits.