= 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.<ADXL335 Data Sheet by Analog Devices>[RevisionB, page10, Mechanical Sensor]{ref}
For an ideal pair of parallel plates with gaps $g-x$ and $g+x$, $C_+-C_-\approx2\varepsilon Ax/g^2$ 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.
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