A 1.8-V 6.9-mW 120-fps 50-Channel Capacitive Touch Readout With Current Conveyor AFE and Current-Driven \Delta \Sigma ADC

• Published in: IEEE journal of solid-state circuits 2018-01, Vol.53 (1), p.204-218
• Main Authors: Hwang, Hyunseok, Lee, Hyeyeon, Han, Myungjin, Kim, Hongchae, Chae, Youngcheol
• Format: Article
• Language: English
• Subjects: Analog to digital conversion; Analog to digital converters; Area-efficient decimation filter; Capacitance; capacitance-to-digital conversion; capacitive touch sensor; Capacitors; CMOS; Conveyors; current conveyor analog front end (AFE); current-driven delta-sigma analog-to-digital converter (ADC); Digitization; energy-efficient; Fingerprinting; Fingerprints; incremental delta-sigma ADC; Integrated circuits; on-glass fingerprint sensor; Oversampling; Prototypes; Signal to noise ratio; Styli; Tactile sensors; Touch screens
• ISSN: 0018-9200; 1558-173X
• DOI: 10.1109/JSSC.2017.2750326

This paper presents an area-and energy-efficient readout for capacitive touch sensors. An analog front end (AFE) with current conveyor mitigates the requirements of frontend opamp and provides a differential current from adjacent channels, removing sensor's baseline and interferences. The sensing current from the AFE is directly digitized by a current-driven 2nd-order ΔΣ analog-to-digital converter and its direct interfacing with the current signal achieves high signal-to-noise ratio (SNR) without suffering voltage saturation in the AFE, and provides reconfigurable SNRs and frame rates with respect to oversampling ratio. An area-efficient sinc 2 filter, whose filter coefficients are commonly provided, enables full parallel implementation of ΔΣ ADCs. A 50-channel prototype IC is fabricated in a 0.18-μm CMOS process, occupying only 1.96 mm 2 . Using a 10.1-in 28 × 50 touch screen panel and a 3.3-V transmitter, this work achieves SNRs of 53.3 and 41.7 dB with finger and 1-mmφ stylus, respectively, while drawing only 6.9 mW from 1.8-V supply at 120 fps. This results in the state-of-the-art figure-of-merit of 0.11 and 0.41 nJ/step for finger and 1-mm-φ stylus, respectively. Moreover, 500-dpi on-glass fingerprint sensor is verified with the same prototype IC and 15-V transmitter, and 70 × 50 fingerprint image is successfully captured with 150-μm cover glass.