A MEMS Coriolis-Based Mass-Flow-to-Digital Converter for Low Flow Rate Sensing

This article presents a microelectromechanical system (MEMS) Coriolis-based mass-flow-to-digital converter ( $\Phi $ DC) that can be used with both liquids and gases. It consists of a micromachined Coriolis mass flow sensor and a CMOS interface circuit that drives it into oscillation and digitizes the resulting mass flow information. A phase-locked loop (PLL) drives the sensor at its resonance frequency ( $f_{D}$ ), while a low 1/ $f$ noise switched-capacitor (SC) proportional–integral (PI) controller maintains a constant drive amplitude. Mass flow through the sensor causes Coriolis-force-induced displacements, which are detected by co-integrated sense capacitors. In-phase ( ${I}$ ) and quadrature ( ${Q}$ ) components of these displacements are then digitized by two continuous-time delta–sigma modulators (CT- $\Delta \Sigma $ Ms) with finite impulse response (FIR)-DACs and passive mixers. Their outputs are used to accurately estimate and cancel sense path delay, thus improving sensor stability. To ensure constant sensitivity over a wide range of fluid densities, a background sensitivity tuning (BST) scheme adjusts the sense capacitors' bias voltage as a function of $f_{D}$ , which is a good proxy for fluid density. Implemented in a standard 0.18- $\mu \text{m}$ CMOS technology, the interface circuit consumes 13 mW from a 1.8-V supply. The proposed MEMS Coriolis $\Phi $ DC achieves a state-of-the-art noise floor of 80 $\mu \text{g}$ /h/ $\sqrt {\text {Hz}}$ and a zero stability (ZS) of ±0.31 mg/h, which is at par with MEMS thermal flow sensors.