AN1711 datasheet(32/40 Pages) STMICROELECTRONICS

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SOFTWARE TECHNIQUES FOR COMPENSATING ST7 ADC ERRORS

Therefore after getting the ADC converted values (Y) we can calculate the actual or ideal an-

alog input signal (x) after knowing the slope of transfer curve (a) and offset constant (b).

Practical measurement

To test the calibration technique, we can use the test setup shown in Figure 20. Analog

Channel 1 is used for gain error calculation and Channel 2 was used for offset calculation. The

test results shown in Table 2. were obtained.

To avoid using floating point variables and the floating point library, all the calibration con-

stants were multiplied by a constant N (= 1000000) and ‘long’ type variables are used.

VREF= 4.9814 V

Table 2. Calibration Results

Comments

1. It can be noted that calibration helps eliminate the errors.

2. In most cases the calibrated value was close to the ideal or expected value.

3. Calibration results are much better in the higher voltage range (>3.5V).

4. For some analog input ranges (approx. 2V to 3V) the recorded values are the same as ex-

pected, and the calibration introduced some errors. This is because of non linearity of the

ADC. We have assumed the actual transfer curve was a straight line, whereas in practice

the ADC transfer curve may exhibit non-linearity.

Vin

Ideal

results

Recorded

value

Calibrated

value

0.509 V

104

102

103

1.005 V

206

205

206

1.502 V

308

309

2.005 V

412

413

412

2.503 V

514

513

3.001 V

616

617

3.504 V

719

720

718

4 V

821

824

821

4.5 V

924

927

924

4.97

1020

1023

1018

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