How to solve the EMI interface to touch screen

Capacitive touch screen as the mainstream technology of the current multi-touch interface is widely used in industrial equipment, it is also called industrial touch screen, anti-interference is one of the performance requirements of the touch screen, if the anti-interference is weak, it will affect the switchboard touch effect, lead to touch insensitive, inaccurate and other problems. The electromagnetic interference problem of industrial touch screen is very challenging in the preliminary design.

The projected capacitive touch screen can pinpoint the position of the finger to touch the screen, and it determines the finger position by measuring small changes in capacitance. A key design issue to consider in such touchscreen applications is the impact of electromagnetic interference (EMI) on system performance. The performance degradation caused by interference may adversely affect the touch screen design, and these interference sources will be discussed and analyzed in this paper.

Projected capacitive touch screen structure

Typical projective capacitive sensors are mounted under a glass or plastic cover plate. The transmitting (Tx) and receiving (Rx) electrodes are connected to a transparent indium tin oxide (ITO) to form a cross matrix, with each TX-RX node having a characteristic capacitance. TITO is located below RxITO and is separated by a polymer film or optical glue (OCA).

Sensor working principle

Let us temporarily ignore the interference factors, to analyze the work of the touch screen: the operator's finger nominal ground potential. The Rx is held at ground potential through the touch screen controller circuit, while the Tx voltage is variable. The varying Tx voltage causes the current to pass through the TX-RX capacitor. A carefully balanced Rx integrated circuit isolates and measures the charge entering Rx, and the measured charge represents the "mutual capacitance" connecting Tx and Rx.

Projective capacitive touch screens, which are widely used in portable devices today, are vulnerable to electromagnetic interference, and interference voltages from internal or external sources can be capacitatively coupled to the touch screen device. These interfering voltages cause charge movement within the touch screen, which can confuse the measurement of charge movement when a finger touches the screen. Therefore, the effective design and optimization of the touch screen system depends on the understanding of the interference coupling path, and to reduce or compensate it as much as possible.

Interference coupling paths involve parasitic effects, such as transformer winding capacitance and finger-device capacitance. Proper modeling of these effects can fully appreciate the source and size of the interference.

For many portable devices, battery chargers constitute a major source of interference with touch screens. When the operator fingers touch the touch screen, the resulting capacitance causes the charger interference coupling circuit to be closed. The quality of the charger internal shield design and whether there is an appropriate charger grounding design are the key factors affecting the interference coupling of the charger.