4. Circuit Guidelines
4.1
More Information
Refer to Application Note QTAN0002, Secrets of a Successful QTouch® Design and the Touch
Sensors Design Guide (both downloadable from the Atmel® website), for more information on
construction and design methods.
4.2
Sample Capacitor
Cs is the charge sensing sample capacitor. The required Cs value depends on the thickness of
the panel and its dielectric constant. Thicker panels require larger values of Cs. Typical values
are 2 nF to 50 nF depending on the sensitivity required; larger values of Cs demand higher
stability and better dielectric to ensure reliable sensing.
The Cs capacitor should be a stable type, such as X±R ceramic or PPS film. For more consistent
sensing from unit to unit, 5 percent tolerance capacitors are recommended. X±R ceramic types
can be obtained in 5 percent tolerance at little or no extra cost. In applications where high
sensitivity (long burst length) is required the use of PPS capacitors is recommended.
For battery powered operation a higher value sample capacitor is recommended (typical value
8.2 nF).
4.3
Power Supply and PCB Layout
See Section 5.2 on page 12 for the power supply range. At 3V current drain averages less than
500 µA in Fast mode.
If the power supply is shared with another electronic system, care should be taken to ensure that
the supply is free of digital spikes, sags, and surges which can adversely affect the QT1010. The
QT1010 will track slow changes in Vdd, but it can be badly affected by rapid voltage fluctuations.
It is highly recommended that a separate voltage regulator be used just for the QT1010 to isolate
it from power supply shifts caused by other components.
If desired, the supply can be regulated using a Low Dropout (LDO) regulator, although such
regulators often have poor transient line and load stability. See Application Note QTAN0002,
Secrets of a Successful QTouch™ Design for further information.
Parts placement: The chip should be placed to minimize the SNSK trace length to reduce low
frequency pickup, and to reduce stray Cx which degrades gain. The Cs and Rs resistors (see
Figure 1-1 on page 2) should be placed as close to the body of the chip as possible so that the
trace between Rs and the SNSK pin is very short, thereby reducing the antenna-like ability of
this trace to pick up high frequency signals and feed them directly into the chip. A ground plane
can be used under the chip and the associated discrete components, but the trace from the Rs
resistor and the electrode should not run near ground, to reduce loading.
For best EMC performance the circuit should be made entirely with SMT components.
10
AT42QT1010
9541G–AT42–03/10
ATMEL [ ATMEL ]
