Design of a Capacitive Humidity Sensor Frontend with an Adaptive Resolution for Energy Autonomous Applications

We present a 9-bit energy-efficient capacitive sensor frontend for the parallel measurement of soil moisture and air humidity with an adaptive resolution. The circuit is based on already published successive approximation (SAR) capacitance-to-digital-converter (CDC) but offers two improvements for range extension specifically for humidity sensing. Commercial sensor elements for humidity sensing show a big variety of dynamic ranges and a considerable large offset capacitance. Hence, a fixed resolution and dynamic range of the CDC would lead to non-optimal range utilization. To overcome this issue, our implementation provides offset subtraction and adaptive range adjustment. The proposed circuit was implemented in a commercial 0.18-mu m CMOS process. Simulations were performed and the CDC achieved an energy per conversion of 11.49 pJ from a 1.8-V supply. The capacitive input range is adjustable in the range of 52 - 825 pF what corresponds to CLSB = 0.1 - 1.61 pF.