at42qt1481 ATMEL Corporation, at42qt1481 Datasheet - Page 13

• Manufacturer Part Number: at42qt1481
• Description: 48-key Qmatrix Ic
• Manufacturer: ATMEL Corporation

2.10
2.11
2.11.1
2.11.2
2.11.3
9621B–AT42–06/11
Key Design
PCB Layout, Construction
Overview
LED Traces and Other Switching Signals
PCB Cleanliness
For information about key design refer to the Touch Sensors Design Guide on the Atmel website
(www.atmel.com/touch).
It is best to place the chip near the touch keys on the same PCB so as to reduce X and Y trace
lengths, thereby reducing the chances for EMC problems. Long connection traces act as RF
antennas. The Y (receive) lines are much more susceptible to noise pickup than the X (drive)
lines.
Even more importantly, all signal related discrete parts (resistors and capacitors) should be very
close to the body of the chip. Wiring between the chip and the various resistors and capacitors
should be as short and direct as possible to suppress noise pickup.
Ground planes and traces should NOT be used around the keys and the Y lines from the keys.
Ground areas, traces, and other adjacent signal conductors that act as AC ground (such as Vdd)
absorb the received key signals and reduce signal-to-noise ratio (SNR) and thus are
counterproductive. Ground planes around keys also make water film effects worse.
Ground planes, if used, should be placed under or around the QT1481 chip itself and the
associated resistors and capacitors in the circuit, under or around the power supply, and back to
a connector, but nowhere else.
Digital switching signals near the Y lines induce transients into the acquired signals,
deteriorating the SNR performance of the QT1481. Such signals should be routed away from the
Y lines, 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 structure (even if on another nearby PCB) should be bypassed to
either Vss or Vdd with at least a 10 nF capacitor of any type, to suppress capacitive coupling
effects which can induce false signal shifts. LED terminals which are constantly connected to
Vss or Vdd do not need further bypassing.
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:
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.
If a PCB is reworked in any way, it is almost guaranteed that the behavior
AT42QT1481
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