small capacitive touch screen 4.3 inch
Developing a mobile handset with a touch screen interface is a complex design challenge, especially for projective capacitive touch screens, which represent the current mainstream of multi-touch interfaces.Projective capacitive touch screens can pinpoint where a finger touches the screen. They measure small changes in capacitance to determine the finger's position.One of the key design issues to be considered in such touch-screen applications is the effect of electromagnetic interference (EMI) on system performance.The performance degradation caused by interference may have an adverse impact on the touch screen design. This paper will discuss and analyze these interference sources.
Projective capacitive touch screen structure
Typical projective capacitive sensors are mounted under glass or plastic covers.Figure 1 shows a simplified side view of the two-layer sensor.The transmitting (Tx) and receiving (Rx) electrodes are connected to a transparent indium tin oxide (ITO) to form a crossover matrix, with each tx-rx node having a characteristic capacitance.Tx ITO is located below Rx ITO and is separated by a polymer film or optical glue (OCA).As shown in the figure, the direction of the Tx electrode is from left to right, and the direction of the Rx electrode is from the outside of the paper to the inside of the paper.
Working principle of sensor
Let's analyze the operation of the touch screen without considering the interference factors: the operator's finger is labeled at the 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. The measured charge represents the "mutual capacitance" connecting Tx and Rx.
Sensor status: not touched
Figure 2 shows a schematic diagram of magnetic field lines without touch.Without finger contact, the tx-rx field lines take up considerable space in the cover.Edge magnetic field lines are projected outside the electrode structure, hence the term "projected capacitance".
Sensor status: touch
When fingers touch the cover plate, magnetic field lines are formed between Tx and fingers, which replace a large number of tx-rx edge magnetic fields, as shown in figure 3.In this way, finger touch reduces the tx-rx mutual capacitance.The charge-measuring circuit recognizes the varying capacitance (delta C) and thus detects the fingers above the tx-rx node.Through the tx-rx matrix of all intersections of the delta C measurement, you can obtain the entire panel touch distribution map.
Figure 3 also shows another important effect: capacitive coupling between the finger and the Rx electrode.Through this path, electrical interference may be coupled to Rx.Some degree of finger-rx coupling is inevitable.
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