A surface micromachined, capacitive accelerometer is described which integrates the mechanical sensing microstructures with CMOS detection circuits. The capacitive sensing structure consists of two polysilicon layers with the sensing and feedback electrodes underneath and the suspended plate as the proof mass. The sensing axis is perpendicular to the substrate. A full capacitive bridge is formed to translate a mechanical displacement signal into an electrical voltage signal.
Electrostatic feedback is used to counteract the proof mass displacement due to acceleration.
Interdigitated fingers are employed to generate levitation force using the asymmetrical distribution of electrical fields. A unity gain buffer with low input capacitance is designed to actively drive the ground plane to minimize the parasitic capacitance. Its digital output is used for the electrostatic feedback so that the feedback force is linearly proportional to the pulse density of the feedback pulse train. Two clock phases are used for separate sensing and feedback in order to eliminate the feedback from the capacitive feed-back. A dual loop design where two identical accelerometers except their ratioed mass are built side by side is employed to cancel the potential drifts from residual stress and environmental changes by subtracting the two digital outputs.