Design of a front-end signal processing circuitry for Capacitive Multi-touch Screens

Abstract

Capacitive Multi-touch Screens are attractive for smart phone, tablet PC, industrial monitors, 3D or 4D visualization interactions, commercial office and so on. With the increasing size of the touch screen, a touch prediction and window sensing (TPWS) strategy was proposed for reducing the system costs and power consumptions. However, the traditional front-end signal processing circuits with charge integrating are not proper for the TPWS strategy. This paper presents a new front-end signal processing circuits dedicated for the TPWS touch screen systems. A charge to digital converter (CDC) is designed to realize the functions of charge integrating circuits and analog to digital converter (ADC) in the traditional touch screen system for the capacitance measurement. Combining differential input sensing and single input sensing, both the capacitance of an intersection and the shunt capacitance of the intersections on one electrode,that is required for the TPWS system, can be measured with the proposed circuits. The design was realized in a 0.13 µm 1.8 V/5 V CMOS process. The area of the prototype chip is 0.087 mm2 per sensing channel. The tests were performed with a 15-inch touch panel. The total power consumption of the system is only about 65 mW with power supply of 5 V, and the reporting rate of the system achieves 500 Hz for none touch and 83 Hz for 10 touches. The proposed circuits satisfy the requirements of the TPWS touch screen systems, and can also be used for the other capacitive touch screen systems.