Electromechanical closed-loop with high-Q capacitive micro-accelerometers and pulse width modulation force feedback

This paper presents a closed-loop interface circuit for high quality factor (Q) capacitive micro-accelerometers. High-Q sensing element is desirable for high resolution, but makes the loop control a great challenge. Considering this, closed-loop implementation utilizing analog force feedback is preferred rather than electromechanical ΔΣ, which is very popular in recent years. To overcome the non-linearity problem of traditional analogue force feedback scheme, pulse width modulation (PWM) force feedback is developed. The chip measures 2.5×2.5mm² in a commercial 0.35μm CMOS process. Using a high-Q capacitive micro-accelerometer, the interface circuit attains an input range of ±1.25g, a white noise equivalent acceleration (NEA) of 17μg/√Hz, a less than 0.1% non-linearity from a single 5V supply, and a good degree of robustness.