How noise affects touch screens

Although not understood explicitly, touch-screens in devices are susceptible to noise. The offending noise sources may be both internal as well as external. Most common sources of noise affecting touch-screens are display and charger noise. Cheap chargers entering the market are inherently noisy, and this affects the functioning of touch-screens. In addition, as devices get thinner, display noise increases.

In addition, many other items of everyday use generate noise that may cause interference. This includes the AC mains, radio signals and the ballasts used for fluorescent lights. When noise is present, low-performance capacitive touch systems may distort the position reported and this may impact the overall system reliability and accuracy.

Injected noise causes large amounts of jitter (highly variable touch coordinates reported for a stationary finger), false touches reported even for no touch on the screen, non-recognition of a finger actually touching the screen and sometimes a complete lock up of the device. For example, noise may prevent you from being able to unlock your phone, since your finger touch is no longer reported or you dial wrong numbers because of jitter and false-touch reporting.

A user experience of touch interface quality is directly dependent on how well a touch-screen controller combats interference from noise. Poor touch performance when noise is present can make customers unhappy, resulting in an increase in returns. However, since noise may be of different types, touch-screen controllers must be able to detect, differentiate and combat noise, especially the two sources most problematic to users – chargers and displays.

The proliferation of Switch Mode Power Supply or SMPS type chargers has reduced the size, weight and cost of mobile chargers. However, this has also led to the market being flooded with chargers that prioritize cost over performance, using lower grade components and not using certain components that would assist in reducing common-mode noise.

High amplitude, high frequency, common-mode noise emanating from chargers is a major problem resulting in degradation of touch performance of capacitive touch-screen devices. Some manufacturers have addressed this problem of noisy chargers by providing limited functionality when a device is plugged into such a charger. Others may show a message on the screen that the charger is not supported when it is not the approved charger for the device. Online forums reveal customer dissatisfaction of touchscreen performance due to noisy chargers is quite prevalent.

Common-mode noise causes fluctuations of both, the power and ground supplies of the charger voltage, relative to earth ground, but keeping the same voltage differential between them. Such fluctuations affect the performance of the touchscreen only when the finger of the user touches the screen. Since the potential of a finger of the user is roughly the same as that of earth ground, and the charger’s ground and power lines are fluctuating relative to it, the resulting noise enters the touchscreen through the finger.

Manufacturers aggressively pursuing thinner form factors for touch-screen devices has led to displays coupling more noise into the touch-sensors because of their proximity. Earlier, touch-screens had an air-gap or a shield layer for protection. With devices getting thinner, such shields and air-gaps have disappeared and the touch sensor is now laminated directly atop the display.
This increases the capacitance, while the sensor electrodes are closer to the noise producing VCOM layer of the display, increasing the coupling.