AN4112

Capacitance measurement

Doc ID 023195 Rev 1

13/17

Figure 10.

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

VDD 1

-tc T

⁄

()

exp

–

()

=

C

t

R

1-threshold

V

DD

()

⁄

()

ln

×

()

⁄

–

=

Ct K

⁄

–

=

K

R

1-threshold

VDD

()

⁄

()

ln

×

=