Rainbow Electronics AT42QT1110-AZ - Manual (Page 8)

8
9570H–AT42–02/10
AT42QT1110-MZ/AT42QT1110-AZ
3. Wiring and Parts
3.1 Cs Sample Capacitors
Cs0 – Cs10 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. Values can be in the range 2.2 nF (for faster
operation) to 33 nF (for best sensitivity); typical values are 4.7 nF to 10 nF.
The value of Cs should be chosen so that a light touch on a key produces a reduction of
~20 to 30 in the key signal value (see
Section 6.8 on page 26 ). 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 – Rs10 are inline with the electrode connections and should be used to
limit electrostatic discharge (ESD) currents and to suppress radio frequency (RF) interference.
Values should be approximately 2 k
to 20 k each; a typical value is 4.7 k.
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 Atmel’s website, www.atmel.com.
3.3 LED Traces and Other Switching Signals
Digital switching signals near the sense lines can 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 1 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.
CAUTION: If a PCB is reworked to correct soldering faults relating to the QT1110, 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.