Low noise capacitive sensor for multi-touch mobile handset's applications

Capacitive touch screen sensor, though it enables multi-touch recognition, has problems rejecting distributed noise such as LCD, RF and EMI interferences coupled to transparent ITO electrodes. This paper presents a capacitive sensor design for multiple channels at the X-Y crossing of ITO patterns on the LCD panel. Capacitance-to-Voltage converter (C-VC) with moving average and time interleaved scheme is proposed to reject noises and achieve high data-rate. Calibration algorithm is used to compensate the delay of each channel due to series RC of ITO pattern. The measured signal-to-noise ratio of the sensor is 24dB at 65Hz scanning frequency for 320 channels. The prototype IC is implemented in a 0.18 CMOS technology, while consuming 13mW from 3.3V and 6mW from 1.8V supply.