An 87×49 mutual capacitance touch sensing IC enabling 0.5 mm-diameter stylus signal detection at 240 Hz-reporting-rate with palm rejection

Abstract

A touch sensing system capable of stylus input should have “palm rejection” function which allows the user to place one's palm on the surface of the touch sensor while writing with a stylus. In order to realize “palm rejection”, it is necessary to detect the small stylus while neglecting large signal from the palm. However this is not sufficient because small electrical noise injected through the palm into the touch sensor impedes stylus input signal in real use. This issue is because the injected noise propagates onto the touch sensor's sense channels which are capacitively-coupled with the palm and degrades the SNR of the stylus signal on the channels. A simple and effective technique to eliminate this issue is implemented in a newly developed 87×49 mutual capacitance touch sensing IC which is fabricated in an 85 nm CMOS technology. It achieves an SNR over 33 dB for a 0.5 mm-diameter stylus when a 10 Vp-p sinusoidal noise is injected to the stylus and the palm. Both the die area and the power consumption of a unit charge-to-voltage converter (CVC) designed for the IC are reduced to approximately 50% compared to those of the previous implementation [4]. In order not to report unwanted touches due to palm signals, a palm detection filter is implemented in the digital signal processor on the IC.