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Know the EMI sources in touchscreens

Posted: 01 Dec 2011     Print Version  Bookmark and Share

Keywords:touchscreen  electromagnetic interference  projected-capacitance 

Designing a mobile handheld device with a touchscreen interface can be a complex design task, especially for projected-capacitance touchscreens, which represent the current mainstream technology for multi-touch interfaces. 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.

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.

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.

 Sensor geometry reference

Figure 1: Sensor geometry reference.

 Flux lines

Figure 2: Flux lines untouched.

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."

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 recognises 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.

 Flux lines

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.

Useful terminologies
Interference 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 interference coupling

Figure 4: LCD Vcom interference coupling model.

LCD Vcom coupling
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 15kHz. Figure 4 shows how the LCD Vcom voltage couples into the touchscreen.

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