High-performance readout circuit for resonator-based MEMS accelerometer using digital control loop

2022-11
Ali, Muhammad
This study proposes a new digital control loop-based compact readout circuit for a resonant MEMS accelerometer providing high performance with reduced temperature and power supply dependence while utilizing low processing power. The readout circuit utilizes a charge-sensing pre-amplifier stage that converts the small motional current to readable voltage, which is then converted to the digital domain using a 16-bit ADC to perform the amplitude and frequency extraction in the digital domain. A Proportional Integral (PI) controller is used to maintain the sensor output at a stable value. The frequency tracking is performed by implementing a Phase-Locked-Loop (PLL) in the digital domain. A timed reference signal technique used in this study reduces the computational requirements for the PLL implementation. A drive signal is generated based on the demodulated frequency and amplitude, and a DAC is used to transfer the signal to the resonators. Chip-level characterization of several resonant MEMS accelerometer sensors is performed with a dynamic signal analyzer (DSA). A lock-in-amplifier is used to characterize different sensor parameters. The frequency-sweeping tests are performed to obtain resonance frequency and gain of the resonators. The PLL and PI controllers, available in the lock-in-amplifier software, are used to operate the sensors in a closed loop. The digital PCB realization is performed using one of the characterized resonant MEMS accelerometers. A frequency-sweeping algorithm is used to obtain the resonance frequency of the resonators under different biasing conditions: 3V to 10V proof mass voltage and 0.1mV to 1mV excitation voltage. The resonance frequency of the resonators is approximately 16485Hz and 16720Hz. The readout circuit accurately and repeatedly measures the resonance frequency of the resonators, and the results are consistent with the sensor characterization results. The frequency-sweeping tests also show that a high value of excitation voltage drives the resonators in the non-linear region. The scale factor calculated with the readout circuit is 95Hz/g which is stable under all the biasing conditions. The sensitivity of the individual resonators is different: 45Hz/g for Resonator 1 and 50Hz/g for Resonator 2. This configuration achieves a bias instability of 9.7µg, which can be further reduced with a better MEMS accelerometer design, and this value is very close to the bias instability obtained using a lock-in-amplifier setup. Testing with a pure sine generator shows that this readout circuit can achieve a 0.3µg/√Hz noise level. The system's bandwidth is approximately 68Hz, which can be improved by sacrificing the noise performance. The temperature compensation improves the bias instability of the sensor by 3 times. The differential resonator design is ideally immune to temperature, but the difference in the sensitivity of the resonators due to fabrication-related problem make the sensor data dependent on the temperature.

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Citation Formats
M. Ali, “High-performance readout circuit for resonator-based MEMS accelerometer using digital control loop,” Ph.D. - Doctoral Program, Middle East Technical University, 2022.