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 front-end 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 Δ Σ 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 sinc2 filter, whose filter coefficients are commonly provided, enables full parallel implementation of Δ Σ ADCs. A 50-channel prototype IC is fabricated in a 0.18- μ mCMOS process, occupying only 1.96 mm2. Using a 10.1-in 28 × 50touch screen panel and a 3.3-V transmitter, this work achieves SNRs of 53.3 and 41.7 dB with finger and 1-mm- φ 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 × 50fingerprint image is successfully captured with 150- μ mcover glass.
Bibliographical noteFunding Information:
Manuscript received May 1, 2017; revised July 4, 2017 and August 28, 2017; accepted August 29, 2017. Date of publication September 26, 2017; date of current version December 26, 2017. This work was supported by the Mid-career Researcher Program through NRF grant funded by the Ministry of Education, Science and Technology (MEST) under Grant 2014R1A2A2A09053061. The work of H. Lee was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (NRF-2017-Fostering Core Leaders of the Future Basic Science Program/Global Ph.D. Fellowship Program). This paper was approved by Guest Editor Joseph Shor. (Corresponding author: Youngcheol Chae.) H. Hwang, H. Lee, M. Han, and Y. Chae are with the Department of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, South Korea (e-mail: firstname.lastname@example.org). H. Kim is with TRAIS, Ansan 425-836, South Korea.
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All Science Journal Classification (ASJC) codes
- Electrical and Electronic Engineering