9.4 A 27µW 0.06mm2 background resonance frequency tuning circuit based on noise observation for a 1.71mW CT-ΔΣ MEMS gyroscope readout system with 0.9°/h bias instability

MEMS gyroscopes are used in closed-loop configuration (CL) to satisfy the demand for high-performance and stable inertial sensors [1]. Due to the higher complexity and power consumption compared to open-loop solutions, these systems have usually been unsuitable for mobile battery-driven devices, e.g., for indoor navigation. Recently, the utilization of CT-ΔΣM for the readout of gyroscopes has shown to be a promising approach for reduced power consumption in a CL system [2]. In general, an accurate matching of the electrical BPF [2] to the drive and sense resonance frequencies of the sensor [3] is a prerequisite for maximizing SNR. For systems with drive frequencies fd of some tens of kHz and with a typical angular rate bandwidth of BW=50Hz, the frequency matching needs to be as precise as BW/fd<0.5%. However, the frequency variation of CT BPFs in CT-ΔΣM over PVT is large compared to DT circuits. This paper presents a fully integrated frequency tuning circuit that is based on noise observation at the input of the electrical BPF in an electromechanical CT-ΔΣM. It works in the background during normal operation, achieving a precision better than 0.25% fd and featuring a considerably lower power of 27µW and lower area of 0.06mm2 than competing approaches (see Fig. 9.4.6).