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.Noise from a display paired with a touch sensor has to be reduced so as not to degrade the high SNR attained with the use of the parallel drive method. A differential sensing scheme between adjacent channels shown in Fig. 12.3.2 is adopted to cancel the strong LCD noise, which is commonly injected to adjacent channels. Original capacitance signals with the common-mode noise rejected are recovered in the digital domain after ADC by summing up the differential signals. A fully differential charge-to-voltage converter (CVC) is designed to have a tolerance against the input common-mode shift caused by the LCD noise and the parallel drive operations. 71 CVCs are laid out in parallel to receive signals from 143 sense channels with a switching operation shown in Fig. 12.3.2. In phase-1 (the first differential signal sensing), the (2i+1)-th and the 2i-th sense channel are connected to the i-th CVC's Inp node and Inn node respectively, while in phase-2 (the second differential signal sensing), the (2i+2)-th and the (2i+1)-th sense channel are connected to the i-th CVC's Inp node and Inn node respectively, where i=0, …, 70. The two phase operations settle in 4μsec, if the time constant of an accompanied touch sensor is small enough.The AFE has 224 sensor channel connections, 143 channel drivers with 3.3V driving voltage, and 71 CVCs. Using the channel switches shown in Fig. 12.3.2, the first 81 channels are connected to the drivers, each of the next 62 channels is connected to either a driver or a CVC via a selectable switch, and the last 81 channels are connected to CVCs. The output voltage signals are transferred from the CVCs to the dual 12b 20MHz pipeline ADCs through two multiplexers, where ADCs take 1.8μs (=36×1/20MHz) to convert all the signals from 71 CVCs. The capacitance distribution of the touch sensor is reconst...

No abstract

No abstract

No abstract

No abstract