A continuous-time ΣΔ modulator with a Gm-C input stage, 120-dB CMRR and −87 dB THD

This paper describes a precision continuous-time ΣΔ modulator for sensor applications. To achieve high input impedance, it uses a G_m-C integrator as its input stage. The non-linearity of this stage is cancelled by a replica transconductor in the modulator´s feedback path, which is driven by a low-pass filtered version of the modulator´s single-bit output. Since the filter adds extra delay, this path is bypassed at high frequencies to preserve modulator stability. The modulator was implemented in a 0.7-μm CMOS process and achieves 79-dB SNR/SNDR and 82-dB DR over a 100-Hz signal bandwidth. It also achieves 32-μV offset, 120-dB CMRR, 120-dB PSRR, and -87 dB THD.