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.

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.