Continuous-time Sigma-Delta modulation for high-resolution, broadband A/D conversion

Although SigmaDelta modulators have largely been implemented as discrete-time (DT) circuits, a continuous-time (CT) design approach offers significant advantages for realizing high-accuracy A/D converters at signal bandwidths where technology considerations may impose significant constraints. A CT approach allows for relaxed amplifier unity-gain frequency and power requirements, which can enable the realization of high-resolution modulators with bandwidths of several MHz or more at low power. It also provides the advantage of inherent anti-aliasing filtering.; This research introduces a hybrid CT/DT SigmaDelta modulator for A/D conversion that combines the benefits of CT and DT circuits, while mitigating the challenges associated with CT design. The proposed architecture enables high-resolution, broadband A/D conversion at moderate power dissipation. The second-order first stage of a two-stage cascade is implemented in CT, while the first-order second stage is a DT circuit. A DT implementation of the second stage dissipates less power than would a CT second stage. Calibration of the integrator time constants in the CT stage reduces mismatch between analog and digital coefficients in the cascade.; The highlights of this research include a detailed analysis of the benefits of CT SigmaDelta modulators and an extensive review of CT design challenges, a proposed design methodology and method of stability analysis, and both an exhaustive exploration of effective CT multibit feedback DAC design and an analysis of the limitations of one such proposed DAC implementation.; An experimental prototype, integrated in 0.18-1.mum CMOS technology, operates from a 1.2-V analog supply to enable straightforward migration to a 0.13-mum or 90-nm CMOS technology. The prototype achieves a dynamic range of 77 dB, a peak SNR of 71 dB, and a peak SNDR of 67 dB for a signal bandwidth of 7.5 MHz and an external sampling clock rate of 240 MHz. The anti-aliasing inherent in the prototype modulator suppresses images that alias down to the signal band edge by 48 dB. The total analog power dissipation is 63.6 mW.