A new differential measurement scheme for projected-capacitive touch controller

For sake of lightness, thinness and low cost, single-layer projected-capacitive touch panel is popular for electronic products. The interference from liquid crystal display in single-layer touch screens is more serious than that in two-layer touch screens. For touch controller IC, differential measurement, which provides high common mode noise rejection, can be used in this case in order to suppress the interference. However, the differential measurement is sensitive to the self-capacitance variation besides the mutual-capacitance variation. As a result, “ghost points” exist in multi-touch application. In order to distinguish the “ghost points”, a new differential measurement scheme is proposed in this paper. By applying complementary excitations on two adjacent drive electrodes, the new measurement scheme only responses to mutual-capacitance variation. A controller IC has been designed based on the proposed scheme in a 0.35 μm 5V CMOS process. The simulation results show that the new scheme effectively solved the problem of “ghost point”. In addition, perfect conversion linearity between mutual capacitance variation and the PGA output voltage is obtained. The touch controller IC in this paper is being fabricated, and the test results are expected in the near future.