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AT42QT1040 [DATASHEET]
9524D–AT42–05/2013
3. Wiring and Parts
3.1 Cs Sample Capacitors
Cs0 – Cs3 are the charge sensing sample ca pacitors; normally they are identical in nomina l value. The optimal Cs
values depend on the corresponding keys electrode de sign, the thickness of the panel and its dielectric constant.
Thicker panels require larger values of Cs. Values can be in the range 2.2 nF (for faster operation) to 22 nF (for best
sensitivity); typical values are 4.7 nF to 10 nF.
The value of Cs should be chosen such that a light touch on a key mounted in a production unit or a prototype panel
causes a reliable detection. The chosen Cs value should never be so large that the key signals exceed ~1000, as
reported by the chip in the debug data.
The Cs capacitors must be X7R or PPS film type, for stability. For consistent sensitivity, they should have a 10%
tolerance. Twenty percent tolerance may cause small differences in sensitivity from key to key and unit to unit. If a
key is not used, the Cs capacitor may be omitted.
3.2 Rs Resistors
The series resistors Rs0 – Rs3 are in line with the electrode connections (close to the QT1040 chip) and are used to
limit electrostatic discharge (ESD) currents and to suppress radio frequency (RF) interference. A typical value is
4.7 k, but up to 20 k can be used if it is found to be of benefit.
Although these resistors may be omitted, the device may become susceptible to external noise or radio frequency
interference (RFI). For details on how to select these resistors refer to Application Note QTAN0002, Secrets of a
Successful QTouch Design, and the Touch Sensors Design Gu ide, both downloadable from the Touch Technology
area of the Atmel website, www.atmel.com.
3.3 LED Traces and Other Switching Signals
For advice on LEDs an d nearby traces, refer to Applica tion Note QTAN0002, Secrets of a Successful QTouch
Design, and the Touch Se nsors Design Guide, both downloadable from the Touch Technology area of Atmel’s
website, www.atmel.com.
3.4 PCB Cleanliness
Modern no-clean flux is generally compatible with capacitive sensing circuits.
3.5 Power Supply
See Section 5.2 on page 15 for the power supply range. If the power supply fluctuates slowly with temperature, the
device tracks and compensates for these changes automatically with only minor changes in sensitivity. If the supply
voltage drifts or shifts quickly, the drift compensation mechanism is not able to keep up, causing sensitivity
anomalies or false detections.
The usual power supply considerations with QT parts apply to the device. The power should be clean and come from
a separate regulator if possible. However, this device is designed to minimize the effects of unstable power, and
except in extreme conditions should not require a separate Low Dropout (LDO) regulator.
CAUTION: If a PCB is reworked to corr ect so lder in g faults re lating to th e device , or
to any associated traces or components, be sure that yo u fu lly und e rst and the
nature of the flux used during the re wo rk pr oc ess . Leakage currents fr om
hygroscopic ionic residues can stop capacitive sensors from functioning. If you
have any doubts, a thorough cleaning after rework may be the only safe option.