A Calibration Technique for The Capacitive Touch Panel Applications

Abstract

The touch screen panel, one of the most popular human-machine interface devices, has been used in many applications, such as in ATMs, notebook PCs, and mobile products. By touching the screen or moving the cursor on it, many functions can be executed more easily without having to use external input devices like a keyboard and a mouse. By adopting the multi-touch recognizable panel, mobile products can have more interesting and complex functions that can be executed anytime and anywhere. For this reason, the touch screen panel technology draws much interest, and a wider market for it is predicted to emerge. There are many sensing methods in existence today, such as the resistive, capacitive, wave, and optical sensing methods. Due to the shielding and averaging effects of the resistive, wave, and optical sensing methods, however, only the projected capacitive method can be used to recognize multi-touch events. In the projected capacitive touch screen panel, however, due to the process variation of the sensing capacitors, a calibration technique is required. The previously reported calibration technique performs calibration by programming the feedback capacitor of the charge amplifier to fix the latter’s output voltages. Every time a capacitor is added to increase the calibration accuracy, the area is doubled and one more frame time is required. Eventually, the chances that an error would arise during calibration increase, and integration becomes very difficult. In the proposed calibration technique, the different output voltages of the charge amplifiers caused by the variation of the sensing capacitors are first sampled, then they are boosted to a reference voltage to extract calibration data. Only one frame time is required to perform the calibration operation, and with the C-2C DAC structure, a higher resolution can be achieved in a small area. The simulation results show that when a 10% variation of the sensing capacitors is given, a less-than-0.5-mV variation is sampled, and the overall calibration operation is completed in only one frame time.