Ultra-Low-Power Front-end Design on Flexible IC Technology for Capacitive Sensors

This paper presents an application-specific integrated circuit (ASIC) front-end, to interface the printed capacitive sensors, using an amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) thin film transistors (TFT) on flexible integrated circuit (FlexIC) technology. A ring oscillator (RO) based front-end sensor interface is designed with 0.024 mm(2) area using 600 nm channel length of n-type TFTs consuming 183 nW at a supply voltage of 1 V. It generates output frequency sensitive to the change in the input capacitance of the printed coplanar sensor introduced by the moisture variations. Measurement results show that the front-end ASIC can detect the change in capacitance from 10 pF up to 500 pF of the printed coplanar sensor making it suitable not only for moisture detection but also for the evaluation of voided liquid volumes inside diapers to realize economical, disposable and battery-less Internet of Things (IoT) sensor nodes using green energy harvested from urine.