Design Considerations
AN64846 - Getting Started with CapSense
®
Doc. No. 001-64846 Rev. *X
82
Figure 3-66. Filter Capacitor Solution for Crosstalk
VDD
LED
CapSense
Sensor
Capacitor
Series
Resistor
CapSense
Controller
LED
Port Pin
Sensor
Port Pin
3.8.10 LEDs Close to CapSense Sensors
If LEDs are placed close to the CapSense sensors (within 4-mm distance), and if either end of the LED changes to a
non-low impedance state at any point in time, the capacitance of the sensors changes between the On and Off states
of the LEDs. The change in output impedance of the LED driver circuit can cause the sensors to false-trigger or sensors
to go off unintentionally when the LEDs change state.
To avoid the effect of LEDs that are placed close to the sensors, the LEDs must be bypassed with a capacitor with a
typical value of 1 nF. This is important in scenarios where LEDs are pulled down or pulled up to switch on and are left
floating when switched off.
The value of the bypass capacitor must be such that it provides a constant low-impedance path as less as 1 k
Ω at
100 KHz, as seen by the sensor on both the ends of the LEDs.
Figure 3-67. LED Circuits
VDD
High or Hi-Z
Low or Hi-Z
3.8.11 Vias
Use the minimum number of vias to route CapSense inputs to minimize parasitic capacitance. The vias should be
placed to minimize the trace length, which is usually on the edge of the sensor pad, as shown in
Figure 3-68. Vias Placement on Sensor Pad
