Cypress CY8C20xx7/S CapSense Series, Design Manual

Page: 27 / 75

CapSense Design Tools
CY8C20xx7/S CapSense
®
Design Guide Doc. No. 001-78329 Rev. *E 27
3.4.2 CSD/CSDPLUS User Module Low-Level Parameters
The CSD/CSDPLUS User Module has several low-level parameters in addition to the high-level parameters. These
parameters are specific to the CSD/CSDPLUS sensing method and determine how raw count data is acquired from
the sensor.
3.4.2.1 I
DAC
Value
The I
DAC
parameter sets the range and sensitivity of the capacitance measurement (the change in raw count per unit
change in C
X
). A higher value of I
DAC
corresponds to a higher range, but a lower sensitivity as explained here.
According to Equation 2-9 , the raw count can be related to C
X
and I
DAC
as follows.
raw count = G
C
C
X
− (2
N
− 1)
I
COMP
I
DAC
Equation 3-2
Where G
C
is the capacitance to digital conversion gain, and is equal to
(2
N
− 1)
V
REF
F
SW
I
DAC
.
Consider a case where compensation I
DAC
value, that is, I
COMP
is zero. In this case, the raw count relation to C
X
( Equation 3-2 ) can be represented by Figure 3-8 .
Figure 3-8. Raw Count versus Sensor Capacitance
raw count
C
X
C
P
C
P
+C
F
C
F
maximum raw count = 2
N
CapSense Signal
0
Slope of the line = G
C
The change in raw count when a finger is placed on the sensor is called a CapSense signal. Figure 3-9 shows how
the value of the signal changes with respect to the conversion gain.