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.

Flow sensors with high resolution (<200g/h/surdHz) and low offset drift (<pm 0.4mg/h) are essential in many microfluidic applications, such as flow cytometry and biological/chemical assays. Although thermal flow sensors can meet these specifications [1], [2], they measure flow velocity, so their calibration is fluid specific. Coriolis flow sensors [3]-[5] are a promising alternative because they measure mass flow and density regardless of fluid type, thus offering more flexibility. However, this has typically been at the expense of lower resolution, offset drift, and large footprint. This paper presents a mass-flow-to-digital converter (phi DC) based on a MEMS Coriolis mass flow sensor and a dedicated readout IC (ROIC). Compared to the state-of-the-art [5], it is more compact and has a digital output. Furthermore, it achieves a 3x improvement in resolution (100 g/h/ surd Hz) and a more than 2 ×improvement in zero stability (pm 0.35mg/h.

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.