We present the design and experimental results of a low-power, high-resolution gyroscope readout circuit. Several techniques are combined to enable an unprecedented level of power savings. Chief among them are automatic mode-matching, positive position sigma-delta force-feedback, and current integrator based position sensing. Mode-matching relaxes the electronic noise budget of the front-end amplifier by the sense Q, resulting in a proportional reduction in front-end power dissipation. Unfortunately, it also results in an extremely narrow open-loop sensor bandwidth owing to the high sense Q, and substantial scale factor and phase uncertainty induced by Q and resonance frequency variations with process and temperature. These problems are overcome in this design by using force feedback to achieve good scale factor stability, a well defined phase relationship to reject quadrature error, and a bandwidth well in excess of 50Hz, commensurate with the requirements of automotive and consumer applications. Unfortunately, with feedback comes the problem of closed-loop stability. High-Q higher-order resonance modes in vacuum packaged devices present additional challenges. The positive position feedback technique addresses this problem and is key to the stable operation of the force-feedback loop. Positive position feedback is combined with the sigma-delta technique to provide inherent A/D conversion. The current integrator based position sense front-end trades the high linearity and gain stability of conventional front-ends for substantially lower power dissipation. The force- feedback loop enables its use without compromising overall system performance since feedback attenuates the error due to the nonlinearity and gain variation of the elements in the loopâ€™s forward path. These techniques were implemented in a 0.35um CMOS ASIC, which dissipates less than 300uA from 3.3V and achieves a Brownian noise limited noise floor of 0.004 deg/sec/rtHz over a 50Hz Band.
Project end date: 01/28/08