A Sub-5mW Monolithic CMOS-MEMS Thermal Flow Sensing SoC With ±6 m/s Linear Range

This article presents a complementary metal-oxide semiconductor (CMOS)-microelectromechanical system (MEMS) monolithic integrated thermal flow sensor system, which consists of a MEMS sensor with dual pairs of thermistors, a precise constant temperature difference (CTD) control circuit, and a low-noise readout circuit with a current feedback instrument amplifier (CFIA). The MEMS sensor is fabricated using an in-house developed post-CMOS process, while its sensing structure is thinned to 2.52 $\mu \text{m}$ for power reduction. Meanwhile, the distance between the microheater and thermistors is optimized with a linear range of larger than ±4 m/s by the Peclet number (Pe) < 1 criterion. The designed CTD control circuit can offer a driving current of 1.88 mA with an output swing of up to 2.82 V, which enables the microheater to operate in 50-K CTD mode with a deviation of less than 0.01 K. Additionally, the designed CFIA has a noise floor of 12.4 nV/rtHz with a 1/f corner of less than 400 mHz. The performance of the system-on-chip (SoC) sensor is evaluated with N2 gas flow. The SoC sensor has a high sensitivity of 156 mV/(m/s) with a detectable flow range of up to ±11 m/s, while its system power is less than 5 mW. The SoC sensor also has state-of-the-art linearity in a range of ±6 m/s and a detection limit down to 86 $\mu \text{m}$ /s. Moreover, the tested results of this SoC sensor are in good agreement with the theoretical models, confirming the feasibility of the proposed design strategy.