A 6-bit Capacitance-to-Digital Converter with 690nJ/c-s FoM Based on Metal Oxide TFTs for Flexible Electronics

Metal Oxide Thin Film Transistors (MO TFTs), known for their flexibility and low cost, are widely used in wearable electronics and biomedical fields. In these applications, capacitive sensors are important for their high sensitivity and low cost, enabling accurate detection of touch and pressure. To make use of these sensor signals, Capacitive-to-Digital Converters (CDCs) are typically used to convert the signals into digital form. Traditional silicon-based CDCs rely on Switched-Capacitor (SC) circuits, with accuracy primarily determined by high-gain Operational Transconductance Amplifiers (OTAs). However, the lack of high-gain OTAs in TFT technique limits SC-based CDC precision. This paper presents a TFT CDC design using an iterative discharge method, avoiding reliance on OTA gain to enhance accuracy and energy efficiency. The proposed design achieves a 6.2-bit Effective Number of Bits (ENOB) for capacitances up to 100pF in 14ms, with a Figure of Merit (FoM) of 690nJ/c-s. To the best of our knowledge, this is the first comprehensive study on TFT CDCs.