Q13Micro and Smart System Technology

Silicon Capacitive Accelerometer

A silicon capacitive accelerometer is a MEMS inertial sensor that measures acceleration by detecting the change in capacitance produced by the displacement of a proof mass (seismic mass) suspended by flexible silicon beams (springs) relative to fixed sense electrodes, when the device experiences acceleration.

In the differential capacitive configuration commonly used, the proof mass sits between two fixed electrodes, forming two capacitors C1 and C2 on either side. Under zero acceleration, the proof mass rests at a central equilibrium position where C1 equals C2. When the device experiences acceleration, inertial force displaces the proof mass relative to the fixed electrodes (in the direction opposite to the acceleration, by Newton's second law applied in the sensor's own non-inertial reference frame), increasing one gap and decreasing the other, so C1 increases while C2 decreases (or vice versa), and this differential capacitance change is converted into a proportional voltage output by an on-chip or associated capacitance-to-voltage conversion circuit (commonly a switched-capacitor charge amplifier).

The differential (push-pull) sensing arrangement provides significant advantages over a single-capacitor design: it doubles the useful signal sensitivity for a given displacement (since both capacitors change in opposite directions), and it inherently cancels common-mode effects such as temperature-induced expansion of the structure and parasitic capacitance drift, since these affect both capacitors equally and are rejected by the differential measurement, leaving only the genuine acceleration-induced differential signal.

The advantages of silicon capacitive accelerometers over alternative sensing principles (such as piezoresistive) include much lower power consumption (capacitive sensing draws negligible DC current, unlike the continuously biased piezoresistive bridge), very low temperature sensitivity and drift (since capacitance depends primarily on geometry rather than a temperature-sensitive material property like resistivity), high sensitivity achievable even with a small proof mass and small displacement, and excellent long-term stability, making capacitive accelerometers the dominant technology used in automotive airbag deployment sensors, smartphone motion sensing, and industrial vibration monitoring applications, despite the additional circuit complexity required for accurate, low-noise capacitance-to-voltage conversion compared to the comparatively simpler piezoresistive Wheatstone-bridge readout.