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AN64846 - Getting Started with CapSense
®
Doc. No. 001-64846 Rev. *X
120
C.
Clearance Between Sensor and
Ground
The ground plane is placed on the same layer of the board as the buttons as shown in
. The clearance
between the button and ground plane plays an important role in the performance of the button. Electric field lines
fringing between a button and the ground plane are illustrated in
. The parasitic capacitance of the sensor,
C
p
, is related to this electric field.
Figure C-1. CapSense Board Top and Bottom Layer
Figure C-2: Button-Ground Plane Fringing Fields
Button
G round
G round
The capacitance, C
P
, decreases as the clearance surrounding the button increases. An example of this dependence of
C
P
through
. In these plots, the board material is FR4 with a thickness of
62 mils (1.57 mm), and the acrylic overlay has a thickness of 2 mm. Each plot contains data for three button sizes (5
mm, 10 mm, and 15 mm diameter).