Active-Passive ΔΣ Modulator for High-Resolution and Low-Power Applications

This paper discusses the use of a low gain amplifier and a passive switched-capacitor (SC) network to enable the SC integrator function. The method is applied to a deltasigma modulator to achieve high resolution as proved by the 65-nm CMOS technology test vehicle. Compared with the conventional operational amplifier (op-amp)-based SC integrator, this solution utilizes a low-gain open-loop amplifier to drive a passive SC integrator with positive feedback. Since the openloop amplifier requires a low dc gain and implements an embedded current adder, the power consumption is very low. Power reduction for single bit is obtained by using passive feedforward with built-in adder to assist the first amplifier. The low swing obtained at the output of the active blocks relaxes the slew rate requirement and enhances the linearity. Implemented in 65-nm digital CMOS technology with an active area of 0.1 mm2, the test chip achieves a dynamic range of 91 dB, peak signal-to-noise ratio of 88.4 dB, peak signal-to-noise-plus-distortion ratio of 88.2 dB, and a spurious free dynamic range of 106 dB while consuming 73.6 μW in a 25-kHz signal bandwidth at 1 V supply, yielding a FoMWalden of 70 fJ/conv-step and FoMSchreier of 176 dB.