Projective Capacitive Touch Screen Structure
Typical projective capacitive sensors are mounted under glass or plastic covers. Figure 1 shows a simplified side view of a double-layer sensor. The emitter (Tx) and receiver (Rx) electrodes are connected to transparent indium tin oxide (ITO) to form a cross matrix. Each Tx-Rx node has a characteristic capacitance. Tx ITO is located below Rx ITO and is separated by a layer of polymer film or optical adhesive (OCA). As shown in the figure, the direction of Tx electrode is from left to right, and the direction of Rx electrode is from outside to inside.
Working Principle of Sensor
Let's analyze the work of the touch screen without considering the interference factor for the moment: the operator's finger is labeled as geopotential. Rx is maintained in the earth potential through the touch screen controller circuit, while Tx voltage is variable. The changing Tx voltage causes the current to pass through the Tx-Rx capacitor. A carefully balanced Rx integrated circuit isolates and measures the charges entering Rx. The measured charges represent the "mutual capacitance" connecting Tx and Rx.
Sensor status: untouched
Figure 2 shows a schematic diagram of the magnetic line in the untouched state. In the absence of finger contact, Tx-Rx magnetic lines occupy considerable space in the cover. The term "projective capacitance" comes from the fact that the edge magnetic lines are projected outside the electrode structure.
Sensor Status: Touch
When the finger touches the cover, magnetic lines form between the Tx and the finger, which replace a large number of Tx-Rx edge magnetic fields, as shown in Figure 3. In this way, finger touch reduces Tx-Rx mutual capacitance. The charge measurement circuit recognizes the varying capacitance (Delta C) and detects the finger above the Tx-Rx junction. By measuring all intersections of Tx-Rx matrix with Delta C, the touch distribution of the whole panel can be obtained.
Figure 3 also shows another important effect: capacitive coupling between fingers and Rx electrodes. Through this path, electrical interference may be coupled to Rx. Some degree of finger-Rx coupling is inevitable.
Special terminology
The interference of the projective capacitive touch screen is generated by the insensitive parasitic path coupling. The term "ground" is usually used to refer to either a reference node in a DC circuit or a low impedance connection to the earth: they are not the same terms. In fact, for portable touch screen devices, this difference is the root cause of touch coupling interference. To clarify and avoid confusion, we use the following terms to evaluate touch screen interference.
Earth: Connect to the earth, for example, through the ground wire of a three-hole AC power outlet. Distributed Earth: Capacitance connection between objects and the earth.
DC Ground: DC reference node for portable devices.
DC Power: Battery voltage for portable devices. Or the output voltage of a charger connected to a portable device, such as a 5V Vbus in a USB interface charger.
DC VCC (DC VCC power supply): A stable voltage supply for portable electronic devices (including LCD and touch screen controllers).
Neutral: AC power circuit (nominal geopotential).
Hot: AC power supply voltage, relative zero line apply electrical energy.
LCD Vcom Coupled to Touch Screen Receiving Circuit
Portable device touch screen can be installed directly on LCD display screen. In a typical LCD architecture, liquid crystal materials are biased by transparent upper and lower electrodes. The lower electrodes determine the number of single pixels of the display screen; the upper common electrodes are the continuous plane covering the entire visual front end of the display screen, which is biased at the voltage Vcom. In a typical low-voltage portable device (such as a mobile phone), the AC VCOM voltage is a square wave oscillating back and forth between DC and 3.3V. AC Vcom level is usually switched once per display line, so the frequency of AC Vcom generated is 1/2 of the product of the refresh rate of display frame and the number of rows. A typical portable device may have an AC Vcom frequency of 15 kHz. Figure 4 illustrates the LCD Vcom voltage coupled to the touch screen.
The two-layer touch screen consists of a separate ITO layer covered with Tx and Rx arrays, separated by a dielectric layer. Tx lines occupy the entire width of the Tx array spacing, and only the minimum spacing required for fabrication is separated between the lines. This architecture is called self-shielding because Tx arrays shield Rx arrays from LCD Vcom. However, coupling may still occur through the interspace of Tx bands.
In order to reduce the cost of architecture and achieve better transparency, single-layer touch screen installs Tx and Rx arrays on a single ITO layer and connects arrays through separate bridges in turn. Therefore, the Tx array can not form a shielding layer between the LCD Vcom plane and the sensor Rx electrode. This may lead to serious Vcom interference coupling.
Charger interference
Another potential source of touch screen interference is the switching power supply for power-fed mobile phone chargers. The interference is coupled to the touch screen through the fingers, as shown in Figure 5. Small cell phone charger usually has AC power supply and zero input, but no ground connection. The charger is securely isolated, so there is no direct current connection between the power input and the secondary coil of the charger. However, this still generates capacitive coupling through switching power supply isolation transformer. The charger interferes with the return path by touching the screen with a finger.
Note: In this case, charger interference refers to the applied voltage of the device relative to the ground. This kind of interference may be described as "common mode" interference because of its equivalence in DC power supply and DC ground. If the switching noise between the DC power supply and the DC ground is not adequately filtered, it may affect the normal operation of the touch screen. This power supply rejection ratio (PSRR) problem is another problem, which will not be discussed in this paper.
Charger Coupling Impedance
The switching interference of charger is generated by the leakage capacitance (about 20pF) coupling of primary and secondary windings of transformer. This weak capacitive coupling can be compensated by parasitic shunt capacitors in the relatively distributed area of charger cables and receiving devices themselves. When the device is picked up, the parallel capacitance will increase, which is usually enough to eliminate the switch interference of the charger and avoid interference affecting the touch operation. When the portable device is connected to the charger and placed on the desktop, and the operator's finger only touches the touch screen, there will be a worst-case interference from the charger.
Switching interference component of charger
The typical mobile phone charger uses flyback circuit topology. The interference waveforms generated by the charger are complex and vary greatly with the charger. It depends on the circuit details and the output voltage control strategy. The interference amplitude also varies greatly, depending on the design effort and unit cost of the manufacturer on the switching transformer shield. Typical parameters include: waveform: including complex PWM square wave and LC ringing waveform. Frequency: 40 ~ 150 kHz under rated load, when the load is very light, the pulse frequency or jump cycle operation drops below 2 kHz. Voltage: up to half of the peak voltage of the power supply = Vrms/2.
Disturbance Component of Charger Power Supply
At the front end of the charger, the AC power supply voltage rectifier generates the charger high voltage rail. In this way, the switching voltage component of the charger is superimposed on a sinusoidal wave half of the power supply voltage. Similar to switching disturbance, the power supply voltage is coupled by a switching isolation transformer. At 50Hz or 60Hz, the frequency of this component is much lower than the switching frequency, so its effective coupling impedance is correspondingly higher. The severity of power supply voltage interference depends on the characteristics of ground parallel impedance and the sensitivity of touch screen controller to low frequency.
Special case of power disturbance: 3-hole plug without grounding Power adapters with higher rated power (such as laptop AC adapters) may be equipped with 3-hole AC power plugs. In order to suppress the output EMI, the charger may internally connect the ground pin of the main power supply to the DC ground of the output. This kind of charger usually connects Y capacitor between the fire line and the zero line and the ground, thus suppressing the conduction EMI from the power line. Assuming that the connection exists intentionally, such adapters will not interfere with portable touch screen devices powered by PCs and USB connections. The dashed wireframe in Figure 5 illustrates this configuration. For PC and its USB-connected portable touch screen devices, if a PC charger with three-hole power input is inserted into an unconnected power socket, a special case of charger interference will occur. Y capacitor couples AC power supply to DC output. A relatively large Y capacitance can effectively couple the power supply voltage, which makes the larger power supply frequency voltage couple through the finger on the touch screen with a relatively low impedance. Summary of this article Nowadays, the projective capacitive touch screen widely used in portable devices is vulnerable to electromagnetic interference. The interference voltage from inside or outside will be coupled to the touch screen device through capacitance. These interference voltages can cause charge movement in the touch screen, which may confuse the measurement of charge movement when the finger touches the screen. Therefore, the effective design and optimization of touch screen system depends on the recognition of interference coupling path and the reduction or compensation as much as possible. The interference coupling path involves parasitic effects, such as transformer winding capacitance and finger-device capacitance. Modeling these impacts appropriately can fully recognize the source and size of interference. For many portable devices, battery chargers constitute the main source of interference for touch screens. When the operator's finger touches the touch screen, the resulting capacitance causes the charger to interfere with the coupling circuit to be turned off. The quality of shielding design inside charger and the proper grounding design of charger are the key factors affecting the interference coupling of charger.
