Electrostatically levitated ring-shaped rotational-gyro/accelerometer using all-digital OFDM detection with TAD

This paper presents a five-axis ring-MEMS rotational-gyro/accelerometer with an all-digital sensor circuit except for a C/V converter. For achieving all-digital construction, we apply Orthogonal Frequency Division Multiplexing (OFDM) using the all-digital TAD-OFDM concept for detecting multi-axis position signals of a single MEMS-ring element (dia. 1.5 mm), which is electrostatically levitated with a PWM-drive force-balance servo-system, using no analog elements after the C/V converter. In this sensor, seven carrier waves are set with specified frequencies arranged in accordance with power-of-two relationship including their π/2-phase-shift waves. Hence, the multiplex-detecting signals can be simultaneously demodulated just by using adders/subtractors. Thanks to this method, we can perfectly separate PWM-drive periods from multi-axis position sensing periods. In this study, the maximally-digital rotational-gyro/accelerometer performance with a 9-bit 25-MS/s TAD in a 0.65-μm CMOS and PWM-drive force-balance technology is experimentally confirmed with FPGA and DSP on the test board, achieving resolutions both 0.1 deg/s/LSB and 1 mG/LSB with a noise level of 0.03 deg/s/Hz^1/2 and 0.3 mG/Hz^1/2, respectively, without the need for any conventional ADCs and DACs.