A programmable mutual capacitance sensing circuit for a large-sized touch panel

This paper presents a programmable capacitive sensing circuit for multi-touch applications, especially enabling mutual capacitance sensing in a large-sized touch panel (over 10-inches). To control the offset of the capacitive sensor, the circuit, based on a switched-capacitor integrator, internally includes a cancelling capacitor array to discharge a feedback capacitor and prevent excessive accumulation. Also, the feedback capacitor array can make the sensitivity of the sensor programmable. With programmable capacitors, this design can make the sensor more compatible with various panels and use a lower feedback capacitor than that of a conventional switched-capacitor. The capacitive interface circuit for a large-sized touch panel has 1728 channels (54×32), which simultaneously sense the capacitance variation during each line driving for mutual capacitance sensing. In addition, charge integration is repeated to increase the signal-to-noise ratio (SNR). The performance of the proposed topology is evaluated mainly with a 10.1-inch LCD touch panel. The measured SNR of the sensor is 23dB at a 60Hz frame rate with twenty repetitions of charge integration. The prototype IC is implemented in a 0.35-um CMOS technology.