A 3.7-mW 12.5-MHz 81-dB SNDR 4th-Order Continuous-Time DSM With Single-OTA and 2nd-Order Noise-Shaping SAR

Abstract

This article presents a hybrid 4th-order delta–sigma modulator (DSM). It combines a continuous-time (CT) loop filter and a discrete-time (DT) passive 2nd-order noise-shaping SAR (NS-SAR). Since the 2nd-order NS-SAR is robust against PVT variation, the stability of this 4th-order DSM is similar to that of a 2nd-order CT-DSM. The CT loop filter is based on single-amplifier–biquad (SAB) structure. As a result, only one OTA is used to achieve 4th-order noise shaping, leading to a high power efficiency. Moreover, this work implements both excess-loop delay (ELD) compensation and an input feedforward path inside the NS-SAR in the charge domain, further reducing the circuit complexity and the OTA power. Overall, this work achieves 81-dB SNDR over 12.5 MHz with 3.7-mW power, leading to a Schreier FoM of 176 dB.