12.3 A 240Hz-reporting-rate 143×81 mutual-capacitance touch-sensing analog front-end IC with 37dB SNR for 1mm-diameter stylus

Realization of a mutual-capacitance touch-sensing system spanning over 30 inches is not a straightforward task, because the SNRs of conventional sequential drive controllers degrade as the number of sensor channels increases. One common way to overcome this drawback is to increase the driving voltage, which however results in an increase in system complexity and cost because it requires high-voltage circuits and devices. This SNR issue is resolved by driving the sensor channels in parallel [1,4] as shown in Fig. 12.3.1. Although the parallel drive mixes up the signals from the multiple channels driven at the same time, the original signals can be reconstructed from the sequence of mixed signals if the drive sequences are linearly independent from each other. By appropriately designing the parallel drive sequences, the SNR is enhanced by √M times compared to that of the sequential drive [1], where M is the number of drive channels. An analog front-end (AFE) IC capable of driving and sensing a 143×81 mutual-capacitance sensor is developed in 0.18μm 1P5M CMOS. A 32-inch and a 70-inch touch system are realized with the use of the AFE and an SNR over 37dB for 1mm diameter stylus is attained in either system.