A 75 dB SNDR 10-MHz Signal Bandwidth Gm-C-Based Sigma-Delta Modulator With a Nonlinear Feedback Compensation Technique

Abstract

Wideband ( $\geq$ 10 MHz) continuous-time (CT) sigma-delta $(\Sigma\Delta)$ modulators generally use active-RC filters. On the other hand, Gm-C-filters avoid the need of a power-hungry driving stage due to their small loading. At the moment, the major challenge for designing Gm-C-based $\Sigma\Delta$ modulator is the narrow linear input range due to the nonlinear Gm amplifier, which leads to low SNDR. This work develops a nonlinearity compensation technique for Gm-C-based $\Sigma\Delta$ modulator. This technique designs a nonlinear feedback DAC which has matched V-I transfer curve as the first Gm amplifier (Gm1). As a result, the distortions of Gm1 are significantly suppressed at the modulator output. This method is prototyped in a 640 MS/s Gm-C-based $\Sigma\Delta$ modulator with 10 MHz signal bandwidth in a 0.18- $\mu{\rm m}$ CMOS process. With this technique, the linear input range of the modulator is greatly enlarged. Consequently, the peak SNDR of the modulator is improved by about 8 dB to 75 dB. This $>$ 12-bit ENOB is achieved for the first time for wide-band Gm-C-based $\Sigma\Delta$ modulators.