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AT42QT1060 [DATASHEET]

9505H–AT42–04/2015

3.

Wiring and Parts

3.1

Cs Sample Capacitors

Cs0 – Cs5 are the charge sensing sample capacitors; normally they are identical in nominal value. The optimal Cs

values depend on the thickness of the panel and its dielectric constant. Thicker panels require larger values of Cs.

Typical values are 2.2 nF to 10 nF.

The value of Cs should be chosen so that a light touch on a key produces a reduction of ~10 – 20 in the key signal

value (see Section 5.22 on page 23). 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. A 20% tolerance may cause small differences in sensitivity from key to key and unit to unit. If a

channel is not used, the Cs capacitor may be omitted.

3.2

Rs Resistors

Series resistors Rs (Rs0 – Rs5) are inline with the electrode connections and should be used to limit electrostatic

discharge (ESD) currents and to suppress radio frequency (RF) interference. They should be approximately 4.7 k



to 20 k

 each.

Although these resistors may be omitted, the device may become susceptible to external noise or radio frequency

interference (RFI). For details of how to select these resistors see the Application Note QTAN0002, Secrets of a

Successful QTouch Design, downloadable from the Touch Technology area of the Atmel website, www.atmel.com.

3.3

LED Traces and Other Switching Signals

Digital switching signals near the sense lines induce transients into the acquired signals, deteriorating the SNR

performance of the device. Such signals should be routed away from the sensing traces and electrodes, or the

design should be such that these lines are not switched during the course of signal acquisition (bursts).

LED terminals which are multiplexed or switched into a floating state, and which are within, or physically very near, a

key (even if on another nearby PCB) should be bypassed to either Vss or Vdd with at least a 10 nF capacitor. This is

to suppress capacitive coupling effects which can induce false signal shifts. The bypass capacitor does not need to

be next to the LED, in fact it can be quite distant. The bypass capacitor is noncritical and can be of any type.

LED terminals which are constantly connected to Vss or Vdd do not need further bypassing.

3.4

PCB Cleanliness

Modern no-clean flux is generally compatible with capacitive sensing circuits.

If a PCB is reworked in any way, clean it thoroughly to remove all traces of the flux residue around the capacitive

sensor components. Dry it thoroughly before any further testing is conducted.

CAUTION: If a PCB is reworked to correct soldering faults relating to the device, or to

any associated traces or components, be sure that you fully understand the nature of the

flux used during the rework process. Leakage currents from 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.