Capacitance measurement
AN3248
14/19
Doc ID 17758 Rev 1
Figure 11.
Capacitance measurement using COMP2
At the moment where the input voltage crosses the threshold and the COMP2 output
switches to high level, the charge function is given by
Equation 1
.
Equation 1
where “tc” is the time when the input voltage crosses the threshold
Using
Equation 1
the capacitance value can be computed by
Equation 2
.
Equation 2
Usually R, the threshold, and V
DD
are constant, so, measuring the capacitance is reduced
to solving for
Equation 3
.
4HRESHOLD
4)-X/#Y
4IME
)NPUTVOLTAGE
4IME
4IME
#APTUREEVENT
4IME
#/-0
OUTPUT
4IMERCOUNTER
#APTUREEVENT
#APTUREEVENT
AI
Threshold
VDD 1
-tc
T
⁄
(
)
exp
–
(
)
=
C
t
R
1-threshold
V
DD
(
)
⁄
(
)
ln
×
(
)
⁄
–
=
