6

9524B–AT42–04/09

AT42QT1040

3.

Wiring and Parts

3.1

Cs Sample Capacitors

Cs0 – Cs3 are the charge sensing sample capacitors; normally they are identical in nominal

value. The optimal Cs values depend on the corresponding keys electrode design, 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 percent 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 inline 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 Guide, both downloadable from the Touch Technology area of Atmel’s website,

www.atmel.com.

3.3

LED Traces and Other Switching Signals

For advice on LEDs and nearby traces, refer to Application Note QTAN0002, Secrets of a

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.

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 in any way, it is almost guaranteed that the behavior

of the no-clean flux will change. This can mean that the flux changes from an inert

material to one that can absorb moisture and dramatically affect capacitive

measurements due to additional leakage currents. If so, the circuit can become

erratic and exhibit poor environmental stability.