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Design Considerations
AN64846 - Getting Started with CapSense
®
Doc. No. 001-64846 Rev. *X
81
Figure 3-63. Routing of Sensing and Communication Lines
3.8.9 Crosstalk Solutions
A common backlighting technique for panels is to mount an LED under the sensor pad so that it shines through a hole
in the middle of the sensor. When the LED is switched on or off, the voltage transitions on the trace that drives the LED
can couple into the capacitive sensor input, creating noisy sensor data. This coupling is referred to as crosstalk. To
prevent crosstalk, isolate CapSense and non-CapSense traces from one another. In the case of CY8C21X34/B, the
crosstalk can also occur due to the coupling of the LED voltage transitions with the R
B
resistor. To avoid this, isolate
the R
B
trace from the non-CapSense traces. A minimum separation of 4 mm is recommended. A hatched ground plane
also can be placed between those traces to isolate them. LED drive traces and CapSense traces (including R
B
trace)
should not be routed together.
Figure 3-64. Not Recommended - LED and CapSense in Close Proximity
Figure 3-65. Recommended - LED and CapSense with Wide Separation
Another approach to reducing crosstalk is to slow down the rising and falling edges of the LED drive voltage using a
filter capacitor.
shows an example circuit of this solution. The value of the added capacitor depends on the
drive current requirements of the LED; however, a value of 0.1 µF is typical.