High performance wideband continuous-time sigma-delta modulator with Gm-C filter

Abstract

Continuous-time sigma-delta modulators (CTSDMs) use oversampling and noise shaping techniques to realize high performance data conversion with a low resolution quantizer. Besides modulator stability, the major design challenges for wideband (≥10MHz) CTSDMs are the implementation of highly linear first stage integrator (INT1) and main feedback digital-to-analog converter (DAC1). So far, almost all wideband CTSDMs use active-RC filters. This architecture needs an input current due to the resistive loading. For high performance applications, a low distortion driving stage is not easy to realize and consumes quite large power. On the other hand, Gm-C filters avoid the need of a driving stage due to their small capacitive loading. Currently, the performance of Gm-C-based modulators is limited by the narrow linear input range due to the nonlinear Gm amplifier, which leads to low SNDR. In this PhD thesis, a novel nonlinearity compensation technique is developed for Gm-C-based modulators, which solves the linearity issue of INT1 and DAC1 simultaneously. The principle is that the V-I transfer curves of the first Gm amplifier (Gm1) and main feedback DAC are designed to match each other so that the DAC has the same nonlinearity as Gm1. As a result, the nonlinearity of Gm1 is compensated and the distortion is greatly suppressed at the modulator output. This method is implemented in a 10MHz bandwidth, 640MS/s CTSDM in 0.18-um CMOS process. With this novel technique, the prototype experimental results show that this modulator achieves 79/77/75dB DR/SNR/SNDR over 10MHz signal bandwidth. This > 12-bit ENOB is achieved first time for wideband Gm-C-based SDMs. The power efficiency is also much improved over previous works. Moreover, the first Gm amplifier is put outside the modulator loop and no longer handles the highly updated (640MS/s) DAC current pulse. Hence, this Gm-C-based architecture excels in another two aspects: better stability and reduced design complexity.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology