A single-mass self-resonating closed-loop capacitive MEMS accelerometer

This paper presents a single-axis, self-resonating accelerometer. The presented accelerometer incorporates a resonating sensing element which is used along with a closed-loop self-resonance circuit, and the analog force-feedback readout circuit. During operation, the sensing element is oscillated at its fundamental frequency through dedicated actuation electrodes in closed-loop configuration. This oscillation is used to modulate the capacitance difference between another set of differential electrodes which are solely used for acceleration sensing. The modulated capacitance difference signal is used to measure acceleration using a closed-loop accelerometer readout circuit. Moreover, the square-wave oscillation signal, which is generated by the self-resonance circuitry, is used to down-convert the modulated acceleration readout signal to baseband. Combining a resonator and an accelerometer in a single mass sensing element, as presented in this work, eliminates the need for an external signal source such as a clock or a function generator which are widely used in capacitive accelerometers. In addition to modulation and demodulation, the oscillation signal can also be benefited in temperature compensation purposes by means of changes in resonance frequency, which is affected by the ambient temperature. Tests held between −20 °C and 60 °C show that there is a strong correlation between the drift in closed-loop accelerometer output and the drift in resonance frequency. By using this correlation, the temperature dependence of the accelerometer output is reduced from −15.4 mg/°C to −76 μg/°C. The noise performance results of the proposed accelerometer indicate the bias instability of 99 μg and the velocity random walk of 54 μg/VHz.