A comparison of dual-slope, switched capacitor, and PWM solid state relative humidity sensor systems

The implementation of a solid-state capacitive relative humidity (RH) sensing system that uses a polymer as the moisture-sensitive material is discussed. The sensing mechanism is based on the fact that the dielectric permittivity of the insulting polymer, polyimide (PI), is linearly related to the ambient relative humidity. Therefore, the capacitance of a parallel plate device using PI as the dielectric is a linear function of ambient RH (because PI does not swell during moisture absorption). Standard IC processing techniques were used to fabricate the integrated sensor capacitor. In order for such a sensor to be commercially viable, a voltage, current, or frequency output is desirable. Several possible circuit implementations of capacitance-to-voltage and capacitance-to-frequency converters optimized for this application have been designed and built (in discrete form). The performance of these systems is reported. These circuits have also been simulated using SPICE. The different system implementations are compared in terms of accuracy, flexibility in sensor capacitor range, linearity, and ease of integration