An LCD-VCOM-Noise Resilient Mutual-Capacitive Touch-Sensor IC Chip With a Low-Voltage Driving Signal

• Published in: IEEE sensors journal 2015-08, Vol.15 (8), p.4595-4602
• Main Authors: Lee, Jae-Seung, Yeo, Dong-Hee, Kwon, Hye-Jung, Kim, Byungsub, Sim, Jae-Yoon, Park, Hong-June
• Format: Article
• Language: English
• Subjects: Capacitive touch sensor; Choppers (circuits); Clocks; Frequency measurement; Logic gates; Noise; referenceless CDR; Tactile sensors; time periodic noise; VCOM display noise
• ISSN: 1530-437X; 1558-1748
• DOI: 10.1109/JSEN.2015.2423556

A mutual-capacitive touch-sensor read-out integrated circuit (ROIC) with a differential sensing scheme is proposed to reduce the driving pulse amplitude of touch sensor panel (TSP) down to 0.4 V. This reduction is achieved by mitigating the effect of the liquid crystal display (LCD) common-voltage (VCOM) noise, which is time periodic. To accomplish this mitigation, the TSP driving pulse (VSTM) is synchronized to the gate-driver-pulse generating signal (GCLK) of LCD, and its frequency is set to (2n + 1) times the frequency of GCLK, where n is a non-negative integer. The repetition period TN of the VCOM noise is one half of the GCLK period. A receiver channel consists of a charge amplifier, a chopper, and an integrator; all are in differential circuits. The chopper is driven by the VSTM. The integration period of the integrator is set to 2T N to suppress the periodic LCD noise. A referenceless clock and data recovery circuit are used to generate VSTM from GCLK continuously with time in spite of the discontinuity of GCLK during the vertical blanking period. The proposed ROIC fabricated in a 0.35-μm CMOS was applied to a 6.9 in 12 × 16 TSP located on a rather noisy 23 in in-plane switching LCD monitor. The measured signal-to-noise ratio changes from 27 to 9 dB as the VSTM amplitude changes from 3.3 to 0.4 V, where the GCLK frequency is 16.8 kHz and the VSTM frequency is 352.8 kHz. The reporting rate of the ROIC is 175 Hz.