A 20bit continuous-time ΣΔ modulator with a Gm-C integrator, 120dB CMRR and 15 ppm INL

This paper describes a precision continuous-time sigma-delta modulator for sensor readout applications. Its input stage consists of a high-impedance Gm-C integrator. The nonlinearity of the input transconductor is compensated for by the inclusion of a low-pass filter and an identical transconductor in the feedback path of the single-bit modulator. The second-order modulator achieves 1 μV offset, 120 dB CMRR and 20-bit resolution over a ±40 mV input range at a conversion rate of 10 S/s. It also achieves 15 ppm INL and less than −100 dB THD for AC signals up to 200 Hz. The modulator is implemented in a 0.7μm CMOS process and draws 240 μA from a 5 V supply.