A projected-capacitance touchscreen is capable of precise touch location based on a light finger touch to the screen. It determines finger position by measuring miniscule changes in capacitance. Developing a mobile handheld device with a touchscreen interface can be a complex design challenge, especially for projected-capacitance touchscreens, which represent the current mainstream technology for multi-touch interfaces.
A key design consideration in this type of touchscreen application is the impact of electromagnetic interference (EMI) on system performance. In this article we’ll explore the sources of interference-caused performance degradation that can negatively impact touchscreen designs and how to mitigate their effects.
Projected capacitance touchscreen geometry
A typical projected-capacitance sensor is assembled to the underside of a glass or plastic cover lens. Figure 1 shows a simplified edge view of a two-layer type sensor. Transmit (Tx) and receive (Rx) electrodes are drawn in transparent indium tin oxide (ITO), forming a matrix of crossed traces, each Tx-Rx junction having a characteristic capacitance. The Tx ITO lies below the Rx ITO, separated by a thin layer of polymer film and/or optically-clear adhesive (OCA). As shown, the Tx electrode runs from left to right, and the Rx electrode runs into the page.
Figure 1: Sensor geometry reference
Sensor normal operation
The operator’s finger is nominally at ground potential. The Rx is held at ground potential by the touchscreen controller circuit, and the Tx voltage is varied. The changing Tx voltage induces current flow through the Tx-Rx capacitance. A carefully-balanced Rx integrating circuit isolates and measures the charge movement into the Rx. This measured charge indicates the “mutual capacitance” linking Tx and Rx.
Sensor condition: not touched
Figure 2 indicates flux lines in the untouched condition. Without a finger touch, the Tx-Rx field lines occupy considerable space within the cover lens. These fringing field lines project beyond the electrode geometry -- thus the term “projected capacitance.”
Figure 2: Flux lines untouched
Sensor condition: touched
When a finger touches the cover lens, flux lines form between the Tx and finger, displacing much of the Tx-Rx fringing field, as shown in Figure 3. In this manner, the finger touch reduces Tx-Rx mutual capacitance. The charge measurement circuit recognizes this changed capacitance (delta C) and the presence of a finger over the Tx-Rx junction is detected. A map of touch across the panel is generated by making delta C measurements at all intersections in the Tx-Rx matrix.
Figure 3: Flux lines touched
Figure 3 demonstrates an important additional effect: capacitive coupling between the finger and the Rx electrode. Through this path, electrical interference may couple onto the Rx. Some degree of finger-Rx coupling is unavoidable.demonstrates an important additional effect: capacitive coupling between the finger and the Rx electrode. Through this path, electrical interference may couple onto the Rx. Some degree of finger-Rx coupling is unavoidable.
Useful terminologyInterference in projected capacitance touchscreens is coupled through parasitic paths that are not entirely intuitive. The term “ground” is commonly used interchangeably in reference to either the DC circuit reference node or a low-resistance connection into planet Earth. These are not the same terms. In fact, for a portable touchscreen device, this difference is the essential cause of touch-coupled interference. To clarify and prevent confusion, we’ll use the following terminology when assessing touchscreen interference.
* Earth – connection to planet Earth, for instance via the earth pin of a 3-pin AC mains socket
* Distributed Earth – capacitive connection of an object to earth
* DC Ground (GND) – DC reference node of a portable device
* DC Power – Battery voltage of a portable device. Alternately the output voltage of a charger connected to the portable device, e.g. 5V Vbus for a USB-interface charger.
* DC VCC – regulated voltage which powers the portable device electronics, including LCD and touchscreen controller
* Neutral – AC mains return, nominally at earth potential
* Hot – AC mains voltage, energized with respect to neutral
LCD Vcom coupling to the touchscreen receive lines
The portable device touchscreen may be mounted directly over an LCD display. In a typical LCD configuration, a liquid crystal material is biased between upper and lower transparent electrodes. The lower electrodes define the individual pixels of the display. The upper common electrode is a continuous plane across the visible front of the display, biased at voltage Vcom. The AC Vcom voltage, as implemented in a typical low-voltage portable device such as a cell phone, is a square wave oscillating between DC ground and 3.3V. The AC Vcom plane typically switches once per display line, so the resultant AC Vcom frequency is one half the display frame refresh rate multiplied by the number of lines. A typical portable device AC Vcom frequency might be 15 kHz. Figure 4 shows how the LCD Vcom voltage couples into the touchscreen.
Figure 4: LCD Vcom interference coupling model
A two-layer touchscreen is implemented with the Tx and Rx arrays on separate ITO layers, spaced by a dielectric layer. The Tx traces occupy the full width of the Tx array pitch, separated only by the minimum trace-trace gap required for manufacturing. This type of construction is referred to as self-shielded because the Tx array shields the Rx array from LCD Vcom. However, there is still potential for coupling to occur through the gaps between Tx strips.
For economy of construction and to achieve better transparency, a single-layer touchscreen implements the Tx and Rx arrays on a single ITO layer, with individual discrete bridges applied to cross one array over the other. As a result, the Tx array does not form a shielding layer between the LCD Vcom plane and the sensor Rx electrodes. This represents a potentially severe Vcom interference coupling situation.
A potential source of touchscreen interference is the switching power supply in a mains-powered cell phone charger. Interference is coupled through the finger to the touchscreen, as shown in Figure 5. Small cell phone chargers typically have AC mains hot and neutral inputs but no earth connection. The charger is safety-isolated, so there is no DC connection between the mains input and the charger secondary. However, there is still capacitive coupling through the switching power supply isolation transformer. The return path for charger interference is through the finger touching the screen.
Note that charger interference in this context is voltage applied to the device with respect to earth. The interference may be described as “common mode” as it appears equally on DC ground and DC power. Power supply switching noise appearing between the charger output DC ground and DC power could be a problem for the touchscreen operation if not adequately filtered. This power supply rejection ratio (PSRR) is a separate issue, which is not addressed in this scenario.
Figure 5: Charger interference coupling model
Charger coupling impedance
Charger switching interference is coupled by the transformer primary-secondary winding leakage capacitance on the order of 20 pF. The effect of this weak coupling is offset by parasitic shunt capacitance to distributed earth occurring in the charger cable and in the powered device itself. Holding the device in the hand applies more shunting, often enough to effectively short the charger switching interference and prevent interference with touch operation. A worst-case charger-generated interference situation occurs when the portable device is connected to the charger and placed on a desktop, and the operator’s finger contacts only the touchscreen.
Charger switching interference component
Typical cell phone chargers use a flyback circuit topology. The interference waveform they generate is complex and varies considerably between chargers, depending on circuit details and output voltage control strategy. The interference amplitude varies considerably depending on how much design effort and unit cost the manufacturer has allocated to shielding in the switching transformer. Typical parameters include:
* Wave shape: complex, consisting of pulse-width modulation square wave followed by LC ringing
* Frequency: 40 – 150 kHz under nominal load, with pulse-frequency or skip-cycle operation dropping frequency to < 2 kHz when very lightly loaded
* Voltage: up to one half mains peak voltage = Vrms / sqrt(2)
Figure 6: Example charger waveform
Charger mains interference component
Inside the charger front end, the AC mains voltage is rectified to generate the charger high voltage rail. As a result, the charger switching voltage component is riding on a sine wave of one half the mains voltage. Similar to the switching interference, this mains voltage is also coupled through the switcher isolation transformer. At 50 or 60 Hz, this component is much lower frequency than the switching frequency, so its effective coupling impedance is proportionally higher. The importance of mains voltage interference depends on the character of shunt impedance to earth and on the touchscreen controller sensitivity to low frequency.
Mains interference special situation: 3-pin plug with missing earth
Power adapters rated for higher power, such as laptop PC AC adapters, may be equipped with a 3-pin AC mains plug. To suppress EMI on the output, the charger will likely have the mains earth pin connected internally through to the output DC ground. Such chargers typically connect Y-capacitors from mains line and neutral to earth to suppress conducted EMI on the mains. Provided the earth connection is present as intended, this type of adapter does not create an interference problem for the powered PC and a USB-connected portable touchscreen device. This configuration is represented by the dotted box in Figure 5.
A special case charger interference situation occurs for a PC and its USB-connected portable touchscreen device if the PC charger with 3-pin mains input is plugged into a mains socket with no earth connection. The Y-capacitors couple the AC mains through to the output DC ground. The relatively large values of the Y-capacitors couple the mains voltage very effectively, resulting in a large mains frequency voltage coupled at relatively low impedance through the finger on the touchscreen.
Projected-capacitance touchscreens commonly used in today’s portable devices are vulnerable to electromagnetic interference. The interference voltages are coupled capacitively from sources that are both internal and external to the touchscreen device. These interference voltages cause charge movement within the touchscreen, which may be confused with the measured charge movement due to a finger touch on the screen. Effective design and optimization of the touchscreen system depends on understanding the interference coupling paths and mitigating or compensating for them as much as possible.
Interference coupling paths involve parasitic effects such as transformer winding capacitance and finger-device capacitance. Proper modeling of these effects yields a detailed understanding of the sources and magnitudes of the interference.
For many portable devices, the battery charger can be a key source of touchscreen interference. The charger interference coupling circuit is closed through the capacitance of the operator’s finger on the touchscreen. The quality of the charger’s internal shielding design and the presence of a proper charger earth connection are key factors affecting charger interference coupling.
About the author:
Vadim Konradi is Staff Applications Engineer, Silicon Labs. He joined Silicon Labs in 2010 as a staff applications engineer, developing human interface devices. Previously, he has worked in companies ranging from startups to large multinational corporations in a variety of technical and management positions focused on occupancy sensors, automatic lighting controls, hospitality Internet, projectors, mainframes and astronaut tools. Mr. Konradi has worked across a variety of technical disciplines including embedded, analog and power electronics, optics, audio and acoustics. His engineering experience includes taking products from concept and requirements through R&D, commercialization, sales, manufacturing and field deployment. He holds a BS in electrical engineering from the University of Texas and an MS in electrical engineering from Berkeley. He has patents in sensors, power conversion and lighting controls.