An efficient DFT-based algoritiim for the charger noise problem in capacitive touch applications

Owing to charger noise, inaccurate touch points and fake touch points may happen so that a device has a wrong behavior. The intensity of charger noise could be much larger than that of original touch signals. Besides, the frequency of charger noise varies for each different charger. Therefore, industry identifies charger noise as the most difficult problem in capacitive touch applications. The solution to the problem has become a key in the mobile market. In this paper, we prove that the combination of frequency hopping and repeated integration is an effective method to handle the problem. In addition, we propose an efficient discrete Fourier transform (DFT)-based algorithm to select a good sampling frequency. Moreover, we show an efficient hardware-software co-design adopting our method in Touch IC. Experimental results show that our method can increase SNR by 37 dB and find a good sampling frequency fast and dynamically.