AN3248

Capacitance measurement

Doc ID 17758 Rev 3

13/17

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

to solving for Equation 3.

Equation 3

where K is solved using Equation 4

Equation 4

Threshold

TIMx OCy

Time

Input voltage

Time

Time

Capture event

0

Time

COMP2

output

65535

Timer counter

Capture event

Capture event

ai18710

Threshold

VDD 1

-tc

T

⁄

()

exp

–

()

=

C

t

R

1-threshold

V

DD

()

⁄

()

ln

×

()

⁄

–

=

Ct

K

⁄

–

=

KR

1-threshold

VDD

()

⁄

()

ln

×

=