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.
All Science Journal Classification (ASJC) codes
- Electrical and Electronic Engineering