AN1711 datasheet(28/40 Pages) STMICROELECTRONICS

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SOFTWARE TECHNIQUES FOR COMPENSATING ST7 ADC ERRORS

3. The process can be automated using external calibration. A precise analog source can be

used for this purpose. The software must make the communication between the source and

the device under test.

4. Store the lookup table in EEPROM

The lookup table for different values can be stored in the connected E2PROM. This will make

it easy to update the lookup table by calibrating from time to time.

This technique can be used in production. A known and precise analog input voltage source of

is used in the test setup and the different voltages are produced. The communication between

test setup and microcontroller provides the digital code that can be expected. The actual dig-

ital code and expected digital code is then used for making the lookup table.

4.2.4 Linear compensation

Linear compensation can be done using the datasheet values for Offset and Gain errors. The

ideal transfer curve is a straight line from code 0 to 1023 for a 10-bit ADC.

The actual ADC transfer curve is assumed to follow a straight line from first transition (digital

code =1) to last transition (digital code = 1023).

The first transition and last transition error values are known from the datasheet. If the Offset

error is positive and the Gain error is negative then the available range is reduced. In this case

the actual transfer curve will cross the ideal transfer curve.

Figure 18. Offset and Gain error compensation

To do linear compensation, the offset and gain errors can be spread over the full range. It can

be noted from the transfer curve that positive offset errors cause the actual result to be re-

VSSA

1023

Digital

output

(Decimal

steps)

Actual

transfer curve

Ideal

transfer curve

Vin

Reduced

Range

Ideal range

Gain error (negative)

Offset

Error (positive)

Offset error positive

Gain error negative

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