A mutual-capacitive touch sensor ROIC using a PLL to reduce LCD noise by synchronizing ROIC TX clock to LCD clock

摘要

A 52.9 dB signal-to-noise ratio (SNR) was achieved in a touch sensor read-out integrated circuit (ROIC) with a 14×16 touch sensor panel (TSP) on a 7.9" liquid crystal display (LCD). Three schemes are proposed in the ROIC to enhance SNR. The first scheme enhances SNR by 14.5 dB by reducing the effect of the LCD display noise; the TSP driving signal (VSTM) frequency is synchronized to (n+0.5) times the gate-driver-pulse generating signal (GCLK) frequency of a LCD by using a digital dual-loop phase-locked-loop (PLL). The second scheme enhances SNR by 4.4 dB by reducing the low frequency noise such as human induced noise; the ROIC output noise is measured with no TSP driving signal at the start of every touch frame and stored in a noise memory, and then the measured noise is subtracted from the measured ROIC output signal during the touch frame. The third scheme enhances SNR by 11.3 dB by employing the delay control of the clock signal that is applied to the chopper circuit of the ROIC receiver.