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SOFTWARE TECHNIQUES FOR COMPENSATING ST7 ADC ERRORS

4 CALIBRATING THE ADC

4.1 CALIBRATION ISSUES

1. If calibration is done on a single channel of the device, the calibration constant is applicable

to all the channels.

2. The ADC conversion errors cannot vary from one channel to another unless the source re-

sistance of any of the analog inputs is different, causing a different voltage drop across the

source resistance. The RAIN (max) value is provided in the ADC electrical characteristics

section of the datasheet.

3. Because of positive offset errors and negative gain errors, the actual range of analog input

voltage range will be reduced. The ADC cannot be calibrated outside this range.

Figure 14. 10-bit ADC Calibration Overview

4.2 CALIBRATION METHODS

The ADC can be calibrated using a known source so as to get accurate results.

4.2.1 Use accurate voltage reference

This is a simple way to calibrate the ADC. A known reference voltage is connected to a free

analog input channel and converted. The digital output received after the conversion can be

compared to the already known correct value. The correction factor is then used to correct all

other the digital values.

Correction factor = known expected value/Actual value

VSSA

1023

Digital

output

(Decimal

steps)

Actual

transfer curve

Ideal

transfer curve

Vin

Reduced

Range

Ideal range

Gain error (negative)

Offset

Error (positive)

Uncalibrated range

Uncalibrated

range

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