Sharp Corp explained the details of its technology that realized a capacitive touch panel having an eight times higher sensitivity (S/N ratio) than before.
The company announced the touch panel Dec 10, 2012, planning to start volume production March 15, 2013.
Sharp improved S/N ratio by about eight times at the time of using a pen on a 20-inch screen by employing a driving method that enables to keep sensitivity even when a screen size increases. In addition, it used a technology for reducing the thickness of a display combined with a touch panel.
In general, when a screen size increases, so does noise from the display, deteriorating the sensitivity of touch panel. To solve this problem, Sharp employed a technology that strengthens signals for detecting touches as a screen size increases. Specifically, it used a parallel driving method in place of a sequential driving method.
With a sequential driving method, signals are read by driving lines one by one. On the other hand, a parallel driving method drives multiple lines at the same time, reads their outputs and obtains signals for touch detection by arithmetic processing.
Because output increases when multiple lines are driven at the same time, the signals generated by arithmetic processing for touch detection become stronger. If the interval between lines of a touch sensor does not change, S/N ratio does not deteriorate even when a screen size increases. With this technology, Sharp developed a touch panel with a size of up to 60 inches.
Furthermore, Sharp employed a technology to reduce the influence of noise so that the thickness of a display combined with a touch panel can be reduced. Noise is generated by a display. And, to reduce the influence of the noise, there is a need to make a space between a display and a touch panel.
To eliminate the need for the space in the aim of reducing thickness, the company applied a low-noise amplifier technology that it developed for TV tuners so that the influence of the noise from a display can be reduced. Noise from a display that writes signals for one line at a time has some characteristics. And, based on them, noise is canceled out.
The new touch panel consists of a touch sensor sheet and a controller chip. The composition of the chip differs depending on screen size. The 5-inch model (for smartphones) has one chip that includes an analog front end, digital front end and CPU. The 7-inch model (for tablet computers) has two chips: a chip containing an analog front end and digital front end and a CPU chip.
The 20-inch model (for monitors and TVs) consists of two chips: an analog front-end chip and a digital front-end chip. And the 60-inch model (for electronic blackboards and table-type personal computers) uses four chips: three analog front-end chips and one digital front-end chip.
The analog front end is used for driving the panel, reading output, amplification and A-D conversion. The digital front end obtains signals for touch detection from output and detects touched areas based on the signals by arithmetic processing.