Solutions to electromagnetic interference of capacitive touch screens

       As the mainstream technology of multi-touch interface, capacitive touch screens are widely used in industrial equipment. They are also called industrial touch screens in the industrial control industry. The anti-interference of capacitive touch screens is one of the performance requirements of touch screens. If the anti-interference is weak, it will affect the touch screen effect of the switchboard, such as insensitive and inaccurate touch. The electromagnetic interference problem of industrial touch screens is extremely challenging in the early development and early design.

      Projected capacitive touch screens can accurately locate the position where a finger touches the screen. It determines the finger position by measuring small changes in capacitance. In such touch screen applications, a key design issue to consider is the impact of electromagnetic interference (EMI) on system performance. The performance degradation caused by interference may have an adverse effect on the touch screen design. This article will discuss and analyze these interference sources.

Projected capacitive touch screen structure

      Typical projected capacitive sensors are installed under a glass or plastic cover. The transmitting (Tx) and receiving (Rx) electrodes are connected to transparent indium tin oxide (ITO) to form a cross matrix, and each Tx-Rx node has a characteristic capacitance. The Tx ITO is located below the Rx ITO, separated by a layer of polymer film or optical adhesive (OCA).

Sensor working principle

      Let's analyze the operation of the touch screen without considering interference factors for the time being: the operator's finger is nominally at ground potential. Rx is maintained at ground potential by the touch screen controller circuit, while the Tx voltage is variable. The changing Tx voltage causes 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.

      The projected capacitive touch screens widely used in portable devices today are very susceptible to electromagnetic interference. Interference voltages from internal or external sources are coupled to the touch screen device through capacitance. These interference voltages cause charge movement within the touch screen, which may 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 its minimization or compensation as much as possible.

      In addition, when designing the touch screen circuit principle, the grounding of the FPC cable can be increased, or the touch screen cable can be covered with double-sided black electromagnetic film, which can also reduce the electromagnetic interference problem of the touch screen in practical applications. Shenzhen Hongjia Technology has been professionally developing and producing 1.14-inch to 10.1-inch displays and supporting touch screens for 12 years. It has a R&D team of more than 20 people who are familiar with solving common problems in the industry and can reduce troubles for customers.

      The interference coupling path involves parasitic effects, such as transformer winding capacitance and finger-device capacitance. Proper modeling of these effects can fully understand the source and magnitude of interference.

      For many portable devices, battery chargers constitute the main source of interference to the touch screen. When the operator's finger touches the touch screen, the capacitance generated allows the charger interference coupling circuit to be turned off. The quality of the charger's internal shielding design and whether there is a proper charger grounding design are key factors affecting the charger's interference coupling.