Developing mobile handheld devices with touchscreen HMIs is a complex design challenge, especially for projected capacitive touchscreen designs, which represent the current mainstream technology for multi-touch interfaces. The projected capacitive touch screen can accurately locate the position where the finger touches the screen, and it can judge the position of the finger by measuring the small change of capacitance. A key design consideration in such touchscreen applications is the effect of electromagnetic interference (EMI) on system performance. Performance degradation caused by interference can have a detrimental effect on touch screen designs, and these sources of interference will be explored and analyzed in this article.
Projected capacitive touch screen structure
Typical projected capacitive sensors are mounted beneath a glass or plastic cover. Figure 1 shows a simplified side view of a two-layer sensor. The transmit (Tx) and receive (Rx) electrodes are connected to transparent indium tin oxide (ITO), forming a cross matrix, with each Tx-Rx junction having a characteristic capacitance. The Tx ITO is located below the Rx ITO, separated by a polymer film or optical glue (OCA). As shown in the figure, the direction of the Tx electrodes is from left to right, and the direction of the Rx electrodes is from the outside of the paper to the inside of the paper
How the sensor works
Let's analyze the operation of the touch screen without considering the interference factors for a moment: the operator's finger is said to be at ground potential. Rx is held at ground potential by the touch screen controller circuit, while Tx voltage is variable. The varying Tx voltage causes current to flow 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 the magnetic force lines in the untouched state. In the absence of finger touch, the Tx-Rx flux lines occupy a considerable amount of space inside the cover. The fringe flux lines are projected out of the electrode structure, hence the term "projected capacitance".
Sensor state: touch
When a finger touches the cover, magnetic force lines are formed between Tx and the finger, and these magnetic force lines replace a large number of Tx-Rx fringing magnetic fields, as shown in Figure 3. In this way, the finger touch reduces the Tx-Rx mutual capacitance. The charge measurement circuit identifies the changing capacitance (△C), thereby detecting a finger over the Tx-Rx junction. By performing △C measurement on all intersections of the Tx-Rx matrix, the touch distribution map of the entire panel can be obtained.
Figure 3 also shows another important effect: the capacitive coupling between the finger and the Rx electrodes. Through this path, electrical interference may couple to Rx. Some degree of finger-Rx coupling is unavoidable.
Terminology
Interference in projected capacitive touchscreens is coupled through imperceptible parasitic paths. The term "ground" is often used to refer to both the reference node of a DC circuit and a low-impedance connection to earth: they are not the same term. 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 touchscreen interference.
•Earth: Connected to the earth, for example, through the ground wire of a 3-pin AC power outlet.
• Distributed Earth (distributed ground): The capacitive connection of the object to the earth.
•DC Ground: DC reference node for portable equipment.
•DC Power: The battery voltage of the portable device. Or the output voltage of a charger connected to a portable device, such as 5V Vbus in a USB interface charger.
• DC VCC (Direct Current VCC Power): A regulated voltage that powers portable device electronics, including LCD and touch screen controllers.
• Neutral: AC power return (nominal at ground potential).
•Hot (Fire): AC mains voltage, applying power relative to neutral.
LCD Vcom is coupled to the touch screen receiving line
Portable device touch screens can be mounted directly to the LCD display. In a typical LCD architecture, the liquid crystal material is biased by transparent upper and lower electrodes. The multiple electrodes below determine the multiple single pixels of the display; the common electrode above is a continuous plane covering the entire visual front of the display, which is biased at the voltage Vcom. In a typical low-voltage portable device such as a cell phone, the AC Vcom voltage is a square wave oscillating back and forth between DC ground and 3.3V. The AC Vcom level is usually switched once per display row, therefore, the generated AC Vcom frequency is 1/2 of the product of the display frame refresh rate and the number of rows. A typical portable device might have an AC Vcom frequency of 15kHz. FIG. 4 is a schematic diagram of the LCD Vcom voltage coupled to the touch screen.
A double-layer touch screen consists of separate ITO layers filled with Tx arrays and Rx arrays, separated by a dielectric layer. The Tx lines occupy the entire width of the Tx array pitch, and the lines are only separated by the minimum pitch required for manufacturing. This architecture is called self-shielded because the Tx array shields the Rx array from the LCD Vcom. However, coupling can still occur through the Tx interband gap.
To reduce architectural cost and achieve better transparency, single-layer touch screens mount Tx and Rx arrays on a single ITO layer, and each array is sequentially spanned by a separate bridge. Therefore, the Tx array cannot form a shield between the LCD Vcom plane and the sensor Rx electrodes. This has the potential for severe Vcom interference coupling.
charger interference
Another potential source of touchscreen interference is the switching power supply of the mains-powered phone charger. Interference is coupled to the touch screen through fingers, as shown in Figure 5. Small cell phone chargers usually have AC line and neutral inputs, but no ground connection. The charger is safety isolated so there is no DC connection between the mains input and the charger secondary. However, this still creates capacitive coupling through the switching power supply isolation transformer. Charger interference creates a return path through a finger touching the screen.
Note: In this context, charger disturbance refers to the applied voltage of the device with respect to ground. This interference may be described as "common mode" interference due to its equivalence at DC power and DC ground. The power switching noise generated between the DC power output of the charger and the DC ground may affect the normal operation of the touch screen if it is not filtered out sufficiently. This Power Supply Rejection Ratio (PSRR) issue is another issue not discussed in this article.
Charger coupling impedance
Charger switching disturbances are coupled through transformer primary-secondary leakage capacitance (approximately 20pF). This weak capacitive coupling effect can be compensated by the parasitic shunt capacitance that appears in the relatively distributed ground of the charger cable and the powered device itself. When the device is picked up, the shunt capacitance will increase, which is usually enough to eliminate the charger switching interference, so that the interference does not affect the touch operation. One of the worst-case interferences from the charger occurs when the portable device is connected to the charger and placed on a table with the operator's fingers only in contact with the touchscreen.
Charger switch interference component
A typical mobile phone charger uses a flyback circuit topology. The interference waveform generated by this kind of charger is more complex and varies greatly with different chargers, depending on the circuit details and output voltage control strategy. Interference amplitudes can also vary widely, depending on the design effort and unit cost invested by the manufacturer in the switching transformer shield. Typical parameters include:
Waveforms: Including complex PWM square waves and LC ringing waveforms. Frequency: 40~150kHz under rated load, when the load is very light, the pulse frequency or skip cycle operation drops below 2kHz. Voltage: up to half of the peak voltage of the power supply =Vrms/√2.
Interference component of charger power supply
On the front end of the charger, the AC mains voltage is rectified to generate the charger high voltage rail. In this way, the switching voltage component of the charger is superimposed on a sine wave of half the supply voltage. Similar to switching interference, this power supply voltage is also coupled through 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 supply voltage disturbances depends on the characteristics of the parallel impedance to ground and also on the sensitivity of the touch screen controller to low frequencies.
Special case of mains interference: 3-pin plug without ground
Power adapters with higher power ratings (such as laptop AC adapters) may be equipped with a 3-pin AC power plug. In order to suppress EMI at the output, the charger may internally connect the ground pin of the main power supply to the DC ground of the output. This type of charger usually connects Y capacitors between the hot and neutral wires and ground to suppress conducted EMI from the power line. Assuming an intentional ground connection exists, this type of adapter will not interfere with powered PCs and USB-connected portable touchscreen devices. The dashed box in Figure 5 illustrates this configuration.
For PCs and their USB-connected portable touchscreen devices, a special case of charger interference occurs if a PC charger with a 3-pin power input is plugged into a power outlet that has no ground connection. The Y capacitor couples the AC power to the DC ground output. A relatively large Y capacitor value couples the supply voltage very efficiently, which allows large supply frequency voltages to be coupled through a finger on the touch screen with relatively low impedance.
Summary of this article
Projected capacitive touch screens, which are widely used in portable devices today, are susceptible to electromagnetic interference, and interference voltages from internal or external sources are capacitively coupled to the touch screen device. These interfering voltages cause charge movement within the touchscreen, which can confound the measurement of charge movement when a finger touches the screen. Therefore, the effective design and optimization of touch screen systems depends on the understanding of the interference coupling path, and its reduction or compensation 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 give a good idea of the source and magnitude of the disturbance.
For many portable devices, the battery charger constitutes a major source of interference on the touchscreen. When the operator's finger touches the touch screen, the generated capacitance makes the charger interference coupling circuit shut down. The quality of the internal shield design of the charger and whether there is a proper charger grounding design are the key factors affecting the interference coupling of the charger.
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