This paper describes an interface circuit for a MEMS Coriolis mass flow sensor that combines both drive and sense circuitry. The MEMS sensor consists of a suspended resonant tube, which is read-out by comb capacitors and driven into oscillation by current flowing through a drive coil in a magnet field. The interface circuit comprises a low-noise front-end that performs capacitance-to-voltage (C/V) conversion, and a drive-loop with automatic amplitude control. Drive motion can also be detected from the output of the front-end, allowing both drive and sense functions to be combined. The front-end is chopped to mitigate its 1/ f noise. When implemented with commercial off-the-shelf (COTS) components, the proposed interface draws 250 mA from a single 5-V supply. Mass flow measurements with Nitrogen gas (N2) show that the sensor's drive frequency drifts by less than 1 mHz (rms) per hour, while its zero stability is less than 2.6 mg/h during an 80s measurement.@en

Precision flow sensors are widely used in the pharmaceutical, food, and semiconductor industries to measure small amounts (<1 gram/hour) of liquids and gases. MEMS thermal flow sensors currently achieve state-of-the-art performance in terms of resolution, size, and power consumption [1, 3]. However, they only measure volumetric flow, and so must be calibrated for use with specific liquids [1] or gases [2, 3]. In contrast, Coriolis flow sensors measure mass flow and thus do not need calibration for specific fluids. Furthermore, their resonance frequency can be used as a measure of fluid density. These features enable significant size, cost, and complexity reductions in low-flow microfluidic systems. Although much progress has been made, miniature [4] and MEMS [5- 7] Coriolis mass flow sensors are still outperformed by their thermal counterparts, especially in terms of resolution and long-term stability.@en

This article presents a microelectromechanical system (MEMS) Coriolis-based mass-flow-to-digital converter (Φ 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 (fD), 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- ΔΣ 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 fD, which is a good proxy for fluid density. Implemented in a standard 0.18- μm CMOS technology, the interface circuit consumes 13 mW from a 1.8-V supply. The proposed MEMS Coriolis Φ DC achieves a state-of-the-art noise floor of 80 μg/h/√ Hz and a zero stability (ZS) of ±0.31 mg/h, which is at par with MEMS thermal flow sensors.@en