Capacitive cmos readouts for high performance mems accelerometers

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2011
Sönmez, Uğur
MEMS accelerometers are quickly approaching navigation grade performance and navigation market for MEMS accelerometer systems are expected to grow in the recent years. Compared to conventional accelerometers, these micromachined sensors are smaller and more durable but are generally worse in terms of noise and dynamic range performance. Since MEMS accelerometers are already dominant in the tactical and consumer electronics market, as they are in all modern smart phones today, there is significant demand for MEMS accelerometers that can reach navigation grade performance without significantly altering the developed process technologies. This research aims to improve the performance of previously fabricated and well-known MEMS capacitive closed loop ΣΔ accelerometer systems to navigation grade performance levels. This goal will be achieved by reducing accelerometer noise level through significant changes in the system architecture and implementation of a new electronic interface readout ASIC. A flexible fourth order ΣΔ modulator was chosen as the implementation of the electro-mechanical closed loop system, and the burden of noise shaping in the modulator was shifted from the mechanical sensor to the programmable electronic readout. A novel operational transconductance amplifier (OTA) was also designed for circuit implementation of the electronic interface readout. Design and fabrication of the readout was done in a standard 0.35 µm CMOS technology. With the newly designed and fabricated readout, single-axis accelerometers were implemented and tested for performance levels in 1g range. The implemented system achieves 5.95 µg/sqrt Hz, 6.4 µg bias drift, 131.7 dB dynamic range and up to 37.2 g full scale range with previously fabricated dissolved epitaxial wafer process (DEWP) accelerometers in METU MEMS facilities. Compared to a previous implementation with the same accelerometer element reporting 153 µg/sqrtHz, 50 µg bias drift, 106.8 dB dynamic range and 33.5 g full scale range; this research reports a 25 fold improvement in noise, 24 dB improvement in dynamic range and removal of the deadzone region.