Using capacitive sensors at long ranges (10-20x their plate diameter)
for long term environmental sensing can be limited by slow but
significant measurement drifts that can often far exceed the small
capacitance variations of interest, which can be around 0.01% or less.
We propose a differential capacitance measurement method that rejects
the quasi-constant drift currents for single plate capacitive sensors by
averaging the absolute slope values of adjacent charge-discharge voltage
ramps of the plate capacitance, under constant current. Compared
analytically and in simulations with period modulation techniques using
astable multivibrators, our method shows much better rejection of drifts
due to quasi-constant charge migration and improved random noise
attenuation, while preserving the measurement sensitivity. We also
provide an implementation example that avoids errors caused by some
types of ramp distortions and improves noise reduction.