Datasheet

12 Document No. 001-66271 Rev. *B CY8C21x34/B CapSense
®
Design Guide
Figure 2-2. Cross Section of Typical CapSense PCB with the Sensor Being Activated by a Finger
D
A
C
r
F
εε
0
=
Equation 1
Where:
C
F
= The capacitance affected by a finger in contact with the overlay over a sensor
ε
0
= Free space permittivity
ε
r
= Dielectric constant (relative permittivity) of overlay
A = Area of finger and sensor pad overlap
D = Overlay thickness
In addition to the parallel plate capacitance, a finger in contact with the overlay causes electric field fringing between
itself and other conductors in the immediate vicinity. The effect of these fringing fields is typically minor compared to
that of the parallel plate capacitor and can usually be ignored.
Even without a finger touching the overlay, the sensor I/O pin has some parasitic capacitance (C
P
). C
P
results from
the combination of the CapSense controller internal parasitics and electric field coupling between the sensor pad,
traces, and vias, and other conductors in the system such as ground plane, other traces, any metal in the product’s
chassis or enclosure, and so on. The CapSense controller measures the total capacitance (C
X
) connected to a
sensor pin.
When a finger is not touching a sensor:
PX
CC =
Equation 2
When a finger is touching the sensor:
FPX
CCC +=
Equation 3
In general C
P
is an order of magnitude greater than C
F
. C
P
usually ranges from 6 pF to 15 pF, but in extreme cases
can be as high as 50 pF. CF usually ranges from 0.1 pF to 0.4 pF. The magnitude of C
P
is of critical importance when
tuning a CapSense system and will be discussed in CapSense Performance Tuning with User Modules.