Capacitance measurement

AN3248

12/17

Doc ID 17758 Rev 3

5

Capacitance measurement

The ability to connect the COMP2 output to the input capture channels of the timers allows

the capacitance value to be measured. The principle is based on measuring the charge time

of a resistor-capacitor (RC) network as follows:

●

the charge time is measured

●

the charge resistor (R) is already known

●

the unknown capacitance (C) can be computed

Figure 10 shows the hardware connection of the RC network to an STM32L1 device.

Figure 10.

RC network connection for capacitance measurement

The capacitance measurement procedure consists of charging and discharging the

capacitor through the resistor. The charge/discharge function follows an exponential curve.

The charge function is given by Example 1.

Example 1

where:

●

●

t is the time

●

T is the RC constant

Charging and discharging the RC network is ensured by the timer output compare channel

(TIMx OC) configured in PWM mode. The timer channel is connected to the resistor.

The input voltage is connected to the COMP2 non-inverting input while the threshold is

connected to the COMP2 inverting input. When the input voltage crosses the threshold, the

COMP2 output switches to high level and a capture event occurs saving the counter value.

Figure 11 shows the capacitance measurement.

C

R

Threshold

TIMx OC

STM32L1 device

Input voltage

-

+

COMP2

TIMx IC

ai18709

Input voltage

V

T

⁄

–

()

exp

–

()

=