Capacitance Discrepancy
Transduction of physical movement into electrical signals relies on the relative variation between two separate capacitor values. Utilizing differential capacitive sensing allows accelerometers and pressure sensors to measure minute displacements of a shared central electrode. When the central electrode shifts closer to one outer plate, the capacitance of that side increases while the opposite side decreases.
Sensing Mechanism
A high-frequency excitation signal is applied to the outer plates to generate a modulated output at the central electrode. Systems employing differential capacitive sensing demodulate this signal to extract the low-frequency physical measurement with high resolution. This ratiometric approach makes the output less sensitive to changes in the supply voltage or excitation frequency.
A differential amplifier then processes the demodulated signal to maximize the signal-to-noise ratio before analog-to-digital conversion.
Symmetrical Configuration
The mechanical layout of the substrate and electrodes is designed to maintain geometric symmetry. Differential capacitive sensing benefits from this balanced design because it inherently cancels common-mode errors. Any uniform change in the dielectric constant or gap size affects both capacitors equally, leaving the differential output unchanged.
Interference Rejection
Stray electromagnetic fields and temperature gradients pose the main threats to measurement accuracy. Using a differential capacitive sensing scheme helps minimize these effects by subtracting the parallel noise signals. This cancellation ensures that only the actual mechanical displacement of the central electrode is recorded.
