 Capacitive accelerometer

A capacitive accelerometer detects acceleration through a change in capacitance caused by motion of a suspended proof mass. In the ADXL335, a micromachined polysilicon structure and differential capacitors form the mechanical sensor and its electrical readout.^[ref]

For an ideal pair of parallel plates with gaps $g-x$ and $g+x$, $C_{+}-C_{-}\approx 2\epsilon Ax/g^2$ when $|x|\ll g$. Combining this with $x\approx -ma/k$ gives an approximately linear acceleration signal. The approximation requires small displacement and operation sufficiently below mechanical resonance. A differential measurement identifies the direction of displacement and rejects some common changes affecting both capacitors similarly.