A ΣΔ modulator for a programmable-gain, low-power, high-linearity automotive sensor interface

Abstract

This paper describes the design and electrical implementation of a 0.35μm CMOS 17-bit≰0kS/s Sigma-Delta Modulator (ΣΔM) forming part of a sensor interface for automotive applications. First of all, the paper discusses the most important limiting factors and design considerations applicable to a high-resolution ΣΔM for sensor interfaces. After an exhaustive comparison among multiple ΣΔM architectures in terms of resolution, speed and power dissipation, a third-order (2-1) cascade ΣΔM is chosen. For a better fitting to the characteristics of different sensor outputs, the ΣΔM here includes a programmable set of gains (0.5, 1, 2, and 4). The gain programmability is implemented by a reconfigurable capacitor array of unitary capacitors. In order to relax the amplifier dynamics requirements for the different modulator gains, switchable capacitor arrays are used for all the capacitors in the first integrator. The design of the modulator building blocks is based upon a top-down CAD methodology which combines simulation and statistical optimization at different levels of the modulator hierarchy. Behavioural simulations considering transistor-level circuit parasitics shows a Dynamic Range (DR) over 105dB for all cases of the modulator gain.