AN2979

Measurements and calibration

Doc ID 15651 Rev 1

9/12

The following figure shows the impact of the capacitor and resistor accuracy.

Figure 7.

Charge equation depending on RC accuracy

1.

2.

Note:

The results are not only impacted by the marging error due to hardware components but

also by the accuracy of the MCU clock. In this example, the device works at a HSI frequency

of 16 MHz at 25 °C, that is, with a clock accuracy of 1% (refer to the datasheet).

The temperature variation also impacts the accuracy of the measured value as the

microcontroller, resistors and capacitors are temperature dependent.

4.3

How to get a better accuracy

4.3.1

Hardware solution

The first solution is to choose components with a high accuracy value (this may imply a

higher cost).

4.3.2

Software solution

reference.

Implementation description

In order to perform the calibration, you need to uncomment the “#define calibration” in the

“param.h” file.

y = 3.3(1-exp(-x/0.882))

y = 3.3(1-exp(-x))

y = 3.3(1-exp(-x/1.122))

0.5

1

1.5

2

2.5

3

0.5

1

1.5

2

2.5

V

ms

Vmeas_max = 2.05

tmax

tacc1max

tacc2max

tacc2max = 1.09 ms

tacc1max = 0.856ms

tmax = 0.970ms