PLL-Based Readout Circuit for SiC-MOS Capacitor Hydrogen Sensors in Industrial Environments

For proper operation in real industrial conditions, gas sensors require readout circuits which offer accuracy, noise robustness, energy efficiency and portability. We present an innovative, dedicated readout circuit with a phase locked loop (PLL) architecture for SiC-MOS capacitor sensors. A hydrogen detection system using this circuit is designed, simulated, implemented and tested. The PLL converts the MOS nonlinear small-signal capacitance (affected by hydrogen) into an output voltage proportional to the detected gas concentration. Thus, the MOS sensing element is part of the PLL's voltage-controlled oscillator. This block effectively provides a small AC signal (around 70 mV at 1 MHz) for the sensor and acquires its response. The correct operation of the proposed readout circuit is validated by simulations and experiments. Hydrogen measurements are performed for concentrations up to 1600 ppm. The PLL output exhibited voltage variations close to those discernable from experimental C-V curves, acquired with a semiconductor characterization system, for all investigated MOS sensor samples.