Electromagnetic Interference and Solutions of Capacitive Touch Screen

Developing a mobile handheld device with a touch screen man-machine interface is a complex design challenge, especially for projected capacitive touch screen design, which represents the current mainstream technology of multi-touch interface.

The projected capacitive touch screen can locate the position where the finger touches the screen. It determines the position of the finger by measuring the small change in capacitance. In this type of touch screen application, a key design issue that needs to be considered is the impact of electromagnetic interference (EMI) on system performance.

Ⅰ. Projected capacitive touch screen structure

A typical projected capacitive sensor is 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. Tx (ITO) is located under Rx ITO, separated by a layer of polymer film or optical adhesive (OCA).

Ⅱ. Working principles of capacitive touch screen sensor

Let us analyze the work of the touch screen without considering interference factors: the operator's hand indicator is 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 flow through the Tx-Rx capacitive touch screen. A carefully balanced Rx integrated circuit isolates and measures the charge entering Rx. The measured charge represents the "mutual capacitance" connecting Tx and Rx.

Without the touch of a finger, the Tx-Rx magnetic field lines occupy a considerable space in the cover. The edge magnetic field lines are projected outside the motor structure, so the term "projected capacitor" comes from it.

Ⅲ. Magnetic lines of force when the capacitive touch screen is not touched

1. Sensor status: touch

When a finger touches the cover plate, magnetic lines of force are formed between the Tx and the finger. These magnetic lines of force replace a large number of Tx-Rx fringe magnetic fields. In this way, the finger touch reduces the Tx-Rx mutual capacitance.

The charge measurement circuit recognizes the changing capacitance (△C), thereby detecting the finger above the Tx-Rx node. Through the △C measurement of all intersections of the Tx-Rx matrix, the capacitive touch screen distribution map of the entire panel can be obtained.

2. Charger interference

Another potential source of touch screen interference is the power supply (switching power supply for mobile phone chargers). Interference is coupled to the touch screen through fingers. Small mobile phone chargers usually have AC power, live wire, and neutral input, but no ground wire connection.

The charger is isolated, so there is no DC connection between the power input and the charger's secondary coil. However, this still produces capacitive coupling through the isolation transformer of the switching power supply. The charger interference forms a return path by touching the capacitive touch screen with a finger.

Projected capacitive touch screens, which are widely used in portable devices today, are susceptible to electromagnetic interference. Interference voltage from internal or external sources will be capacitively coupled to the touch screen device.

These interference voltages will cause charge movement in the capacitive 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 depend on the understanding of the interference coupling path. And to reduce or compensate for it as much as possible. The interference coupling path involves derivative effects, such as transformer-winding capacitance and finger-device capacitance. Proper modeling of these effects can fully recognize the source and magnitude of the interference.

For many portable devices, battery chargers constitute the main source of interference for touch screens. When capacitive touch screen manufacturers touch the touch screen, the generated capacitance causes the charger to interfere, and the coupling circuit is closed.

The quality of the internal shielding design of the charger and the proper grounding design of the charger are the key factors that affect the interference coupling of the charger.