A Gm-C Continuous-Time Sigma-Delta Modulator with Improved Linearity

Abstract

Bridge sensors are widely used for accurate measurement of physical quantities such as temperature, pressure, strain or altitude. Such sensors require a low-noise, high-resolution and accurate readout system with high input impedance. In order to meet these requirements, conventional sensor readout systems use multiple stages which typically include a low-noise preamplifier, an anti-aliasing filter and a discrete-time (DT) sigma-delta modulator (??M). As a result, these systems involve several high-gain loops with total open-loop gain far exceeding the required closed-loop gain. This can lead to sub-optimal power dissipation and greater analog design complexity in design of a sensor readout system. In recent years, Gm-C continuous-time (CT) ??Ms have attracted a lot of attention due to their inherent anti-alias filtering, low power dissipation, high input impedance and high resolution. However, their use in precision applications such as bridge sensor readout is limited by the nonlinearity of the input stage. In this work, a new single-bit CT??M topology is proposed that employs an identical nonlinear element in the feedback path along with a low pass filter to enable nonlinearity compensation and achieve high linearity. A feedforward Gm stage further enhances the nonlinearity compensation by increasing the effective loop-gain. This approach enables more than 60 dB improvement in the nonlinearity of the input transconductor stage of the CT??M. A precision sensor readout circuit using the proposed CT??M architecture is designed and implemented in 0.7 µm technology. The modulator achieves a resolution of 20 bits with a 22 nV/?Hz noise floor and an accuracy better than 10 ppm in post-layout simulations. It consumes 240 µA current from a 5 V supply. The resolution and accuracy of the CT??M designed in this work is comparable to that of state-of-the-art readout systems but with lower power dissipation and lesser analog complexity. The proposed modulators achieves 10x better linearity and accuracy compared to the state-of-the-art Gm-C based CT??Ms, albeit at low frequencies, with significantly less noise and power dissipation.