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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.

A microelectromechanical system combines small mechanical structures with electrical sensing or actuation. Fabrication methods related to semiconductor device fabrication can create suspended beams, membranes and movable masses together with their electrodes. Sensors and actuators can consequently be integrated into compact devices.

A capacitive accelerometer illustrates the mechanism. A suspended proof mass moves relative to its support under acceleration. In a simple spring model, a slowly varying support acceleration $a$ produces a relative displacement $x\approx -ma/k$, where $m$ is the mass and $k$ is the spring constant. Measuring a change of capacitance then measures that displacement. The ADXL335 uses suspended polysilicon structures and a differential capacitor to turn motion into an electrical signal.^[ref]

For an idealized pair of parallel-plate capacitors with equal area $A$ and gaps $g\mp x$, the capacitance difference isThis model explains why a small mechanical displacement can produce a signed electrical measurement; it is not a detailed model of every commercial MEMS geometry.

A proof mass is an inertial element whose relative motion or required support force provides a measure of acceleration. In a spring-supported accelerometer, the instrument frame moves with the object being measured, while the proof mass responds through its inertia and suspension.

An ideal one-axis model is $m\ddot{x}+c\dot{x}+kx=-ma$, where $x$ is displacement relative to the frame, $m$ is the proof mass, $c$ is damping and $k$ is the spring constant. At frequencies well below resonance, $x\approx -ma/k$. A capacitive accelerometer measures this displacement electrically. The suspension determines sensitivity and bandwidth together; a more compliant spring gives greater static displacement but a lower natural frequency.^[ref]