APPLICATION OF CODE DIVISION MULTIPLE ACCESS TECHNOLOGY IN READOUT CIRCUIT AND SYSTEM DESIGN FOR AN ULTRA-THIN ON-CELL FLEXIBLE CAPACITIVE TOUCH PANEL

A capacitive sensing circuit including electrodes for a 7-inch ultra-thin flexible on-cell touch panel has been designed. Implementing code-division multiple sensing (CDMS) with Walsh transform to scan Tx electrodes is chosen to improve the signal-to-noise ratio (SNR). The algorithm applies to field programmable logic array (FPGA). The sensing readout algorithm is applied to work on 4 Tx transmitter electrodes and 4 Rx sensing electrodes. The switched-capacitor (SC) circuit is applied to avoid disturbing sample signal from parasitic capacitance and enlarge the voltage difference from capacitance changes of the touch panel. 12-bit ADC to transfer the front-end analog signal to digital code. The digital part adopts a correction algorithm to eliminate the background value of the panel, the moving average algorithm has an adjustable signal-to-noise ratio function, and the Walsh conversion demodulation algorithm improves the touch report rate to achieve high SNR with up-to 34 dB.