Multi-Bit Capacitance Sensing System Using a-IGZO TFT Technology for Smart Wearables

Abstract

This brief presents a novel multi-bit Capacitance-to-Digital converter (CDC) using unipolar single-gate amorphous-Indium-Gallium-Zinc-Oxide thin-film transistors (a-IGZO TFTs). This circuit is fabricated on a $30{\mathrm {\,}} \mu $ m thick polyimide substrate with an active area of $6.5{\mathrm {\,}}$ mm2. The proposed CDC is designed by employing Charge-Sharing Successive-Approximation Register Analog-to-Digital Converter (CS SAR ADC). Further, the design facilitates integration of capacitance sensor/array directly with the ADC, hence the additional interfacing circuits between the capacitive-sensor and the ADC can be eliminated to make the system compact and energy-efficient. The functionality of the proposed CDC is demonstrated for a sensor capacitance value ranging from $1{\mathrm {\,}}$ pF to $31{\mathrm {\,}}$ pF. From measurements it is observed that the minimum value of capacitance that can be detected with the proposed CDC is around $2{\mathrm {\,}}$ pF, while the state-of-the-art CDC is around $3.7{\mathrm {\,}}$ pF, which is reported on a truly flexible substrate. Further, the ADC deployed in the CDC has resulted in an SNR of $35.57{\mathrm {\,}}$ dB, figure-of-merit (FoM) of $19.9{\mathrm {\,}}$ nJ/c.s., ENOB of $5.6{\mathrm {\,}}$ bits, differential non-linearity (DNL) of $0.52{\mathrm {\,}}$ LSB and an integral non-linearity (INL) of $0.81{\mathrm {\,}}$ LSB. At a sampling frequency of $2.08{\mathrm {\,}}$ kHz, the ADC has shown a total power dissipation of $2.02{\mathrm {\,}}$ mW with a supply voltage $(V_{DD})$ of $4{\mathrm {\,}}$ V. This capacitance sensing system finds potential applications in areas of biomedical, healthcare, and smart packaging systems etc, which need truly flexible devices.