AN2979 datasheet(8/12 Pages) STMICROELECTRONICS

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Measurements and calibration

AN2979

8/12

Doc ID 15651 Rev 1

Measurements and calibration

4.1

Typical measurements

The following values are given for information only.

4.2

Precision of the measured value

When using the RC charge equation, the precision of the measured value depends on the

accuracy of the capacitor C and the resistor R.

Accuracy example:

if C=100 nF with 10% accuracy: Cacc1 =90 nF or Cacc2 =110 nF

if R=10 k

Ω with 2% accuracy: Racc1=9.8 kΩ or Racc2=10.2 kΩ

Calculation of the RC constant:

Charge equations:

Table 1.

Measured value after conversion and margin error for VDD=3.3 V

Value to be measured (V)

Measured value after conversion (V)

Error (%)

0.25

0.204

18.4%

0.5

0.419

16.2%

0.8

0.704

12%

0.903

9.7%

1.25

1.14

8.8%

1.5

1.39

7.33%

1.75

1.628

6.97%

()

100

–

()

1 ms

ACC1

9.8

()

–

()

882

μs

ACC2

10.2

()

110

–

()

1.122 ms

meas

---

–

⎝⎠

⎛⎞

exp

–

⎝⎠

⎛⎞

measacc1

acc1

-----------------

–

⎝⎠

⎜⎟

⎛⎞

exp

–

⎝⎠

⎜⎟

⎛⎞

measacc2

acc2

-----------------

–

⎝⎠

⎜⎟

⎛⎞

exp

–

⎝⎠

⎜⎟

⎛⎞

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