Capacitive Pressure Sensing: Geometry and Readout

A capacitor’s value depends on its geometry and dielectric. If pressure deflects a diaphragm or compresses a dielectric, the resulting capacitance change can be measured. This article describes the design principle; it does not report a calibrated sensor test.

The Governing Physics

Capacitance of a parallel-plate capacitor:

C = ε₀ × εᵣ × A / d

Where:

A — plate overlap area (m²)
d — plate separation (m)
εᵣ — dielectric permittivity

With a fixed plate area and dielectric, reducing a 10 µm gap to 9 µm raises the ideal parallel-plate capacitance by about 11%. Fringing fields, diaphragm shape, and material changes make the response of a real sensor less simple.

Two Structural Approaches

1. Diaphragm-Type (MEMS Style)

A thin conductive membrane suspended over a fixed plate. Applied pressure deflects the membrane, decreasing the air gap:

ΔC / C₀ ≈ -Δd / d₀    (for small changes in gap)

MEMS devices use this principle at micrometre scale. A flexible electrode over a cavity is one possible prototype approach, but its pressure range and sensitivity depend on the mechanical design and must be calibrated.

2. Dielectric Compression

Soft dielectrics between plates may compress under load. Both gap and permittivity can change, so the capacitance-to-pressure relationship needs measurement and calibration.

Readout Circuit

One readout option converts capacitance to an oscillator frequency; the frequency equation depends on the circuit used. A capacitance-to-digital converter is another option. For example, the AD7746 data sheet specifies a ±4 pF changing-capacitance range and up to 17 pF of common-mode capacitance. Its 4 aF resolution figure describes noise under specified conditions, not overall measurement accuracy. The sensor and readout must be matched for baseline capacitance and expected change.

Noise and Stability Concerns

Parasitic capacitance: Keep sense connections short and consider driven guards or shielding where the readout supports them.
Humidity: Moisture can change dielectric properties and leakage; packaging choices depend on the intended environment.
Temperature drift: Characterize the sensor and readout across the operating range, then calibrate or compensate as needed.

Practical Build Notes

A prototype could use two electrodes separated by a flexible film. The next steps would be to measure baseline capacitance, apply known pressures, repeat loading cycles, and check drift and hysteresis. No pressure-to-capacitance measurement is reported here.

The geometry sets the ideal sensitivity; calibration and environmental testing determine whether a particular build is useful as a pressure sensor.