In a touchscreen system, the projected capacitive touchscreen sensor sits on top of the display. Displays are inherently noisy, which can cause display noise to directly couple with the touchscreen sensor.This diminishes touch sensitivity and produces false touch activation. Good design choices can mitigate display noise and have a substantial impact on performance and cost.
To block display noise, the industry traditionally implements an additional ITO “shield” layer between the display and touchscreen sensor. Though effective, the shield layer adds cost and increases the thickness of the touchscreen module. An alternative is using a tiny air gap, typically between 0.2 mm and 0.5 mm, to separate the display from the touchscreen sensor.
An air gap is more cost-effective than a shield layer, but it also increases the touchscreen module thickness, which is becoming undesirable to OEMs looking to build sleeker and thinner devices. A more important design choice will be the selection of the display itself.
Currently the most popular displays used for mobile phones and tablets are still thin-film transistor (TFT) LCDs, which are commonly available in two flavors: dc common voltage (DCVCOM) and ac common voltage (ACVCOM). The difference is the method used to drive the common electrode layer (VCOM). Another increasingly popular display in high-end devices is the active-matrix organic LED (AMOLED) with its wide viewing angles, improved brightness and contrast, lower power consumption, and reduced thickness.
AMOLEDs emit very little display noise and are among the quietest displays, but they are expensive. DCVCOM is also generally a quiet display and expensive. In contrast, ACVCOM is high-noise but relatively cheap. The choice of a display greatly relies on the device’s intent for end customers. The target application will deem the hardware and performance that is suitable for its customers.
