User guide
Getting Started with CapSense
®
Document No. 001-64846 Rev. *I 11
2.1.2 Mutual-Capacitance
Mutual-capacitance uses a pair of pins and measures the capacitance between those pins. A mutual-capacitance
system operates by driving a current on a transmit pin and measuring the charge on a receive pin. When a finger is
placed between the transmit and receive pins, it decreases the measured capacitance. The mutual-capacitance effect
is best suited to multitouch systems, such as touchscreens and trackpads.
Cypress‘s TrueTouch
®
touchscreen solutions use mutual-capacitance sensing. See TrueTouch Touchscreen
Controllers to learn about these products. Cypress also offers trackpad solutions. Contact your local Cypress sales
office directly for more information. To find your local sales office, click here.
2.2 Self-Capacitance Equivalent Model
In a CapSense self-capacitance system, the sensor capacitance measured by the controller is called C
X
. When a
finger is not on the sensor, C
X
equals the parasitic capacitance of the system. This parasitic capacitance, C
P
, is a
simplification of the distributed capacitance that includes the effects of the sensor pad, the overlay, the trace between
the CapSense controller pin and the sensor pad, the vias through the circuit board, and the pin capacitance of the
CapSense controller. C
P
is related to the electric field around the sensor pad. Although Figure 2-3 shows field lines
only around the sensor pad, the actual electric field is more complicated.
Figure 2-3. C
P
and Electric Field
When a finger touches the sensor surface, it forms a simple parallel plate capacitor with the sensor pad through the
overlay. The result is called finger capacitance, C
F
, and is defined by Equation 1. C
F
is a simplification of a distributed
capacitance that includes the effects of the human body and the return path to the circuit board ground.
Equation 1
Where:
ε
0
= Free space permittivity
ε
r
= Dielectric constant of overlay
A = Area of finger and sensor pad overlap
D = Overlay thickness