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How to Minimize Touchscreen Electromagnetic Interference
Developing a mobile handheld device with a projected-capacitance touchscreen interface can be
a complex design challenge. A projected-capacitance touchscreen, which represents the current
mainstream technology for multi-touch interfaces, enables precise touch location based on light
finger contact with the screen. The touchscreen 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. Let’s
explore the sources of interference-caused performance degradation that can negatively impact
touchscreen designs.
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 with 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
Ignoring interference for the moment, let’s analyze touchscreen operation as follows: 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
Silicon Laboratories, Inc. Rev 1.0 1 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 shows 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 thereby 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.
Silicon Laboratories, Inc. Rev 1.0 2 Figure 3. Flux Lines Touched
As shown in Figure 3. Flux flux 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 Terminology
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 the earth ground. 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, let’s use the following terminology
when assessing touchscreen interference.
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Earth – connection to earth ground, 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. 5 V Vbus for a USB-interface charger.
DC VCC – Regulated voltage that 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.
Silicon Laboratories, Inc. Rev 1.0 3 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 3.3 V. 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 is coupled to 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.
Charger Interference
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” since 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 touchscreen operation if not adequately filtered. This power supply
rejection ratio (PSRR) is a separate issue, which is not addressed in this scenario.
Silicon Laboratories, Inc. Rev 1.0 4 Figure 5. Charger Interference Coupling Model
Charger Coupling Impedance
Charger switching interference is coupled by the transformer primary-secondary winding leakage
capacitance (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:
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Wave shape: complex, consisting of pulse-width modulation square wave followed by LC
ringing
Silicon Laboratories, Inc. Rev 1.0 5 
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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 at a 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 frequencies.
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 Ycapacitors from the mains line and are 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.
Silicon Laboratories, Inc. Rev 1.0 6 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.
Summary
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.
Silicon Laboratories, Inc. Rev 1.0 7