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AN2834

ADC errors

This section lists the main errors which have an effect on A/D conversion accuracy. These

types of error occur in all A/D converters and conversion quality depends on eliminating

them. Values for these errors are specified in the ADC characteristics section of the STM32x

device datasheets.

Different accuracy error types are specified for the STM32x ADC. Accuracy errors are

normally expressed as multiples of 1 LSB for easy reference. The resolution in terms of

voltage depends on the reference voltage. The error in terms of voltage is calculated by

multiplying the number of LSBs with the voltage corresponding to 1LSB (1LSB = VREF+/212

or VDDA/212).

2.1

ADC errors related to the ADC itself

2.1.1

Offset error

It is defined as the deviation between the first actual transition and the first ideal transition.

The first transition occurs when the digital output of the ADC changes from 0 to 1. Ideally,

when the analog input is between 0.5 LSB and 1.5LSB, the digital output should be 1.

Ideally still, the first transition occurs at 0.5 LSB. The offset error is denoted by EO. Offset

error can easily be calibrated by the application firmware.

Example

For the STM32x ADC, the smallest detectable incremental change in voltage is expressed

in terms of LSBs:

1 LSB = VREF+/4096 (on some packages is VREF+ = VDDA).

If VREF+ = 3.3 V, ideally, the input of 402.8 µV (0.5 LSB = 0.5 × 805.6 µV) should lead to the

generation of a digital output of 1. In practice, however, the ADC may still show the reading

as 0. If a digital output of 1 is obtained from an analog input of 550 µV, then:

Offset error = Actual transition – Ideal transition

EO = 550 µV – 402.8 µV = 141.2 µV

EO = 141.2 µV / 805.6 µV = 0.17 LSB

When an analog input voltage greater than 0.5LSB generates the first transition, the offset

error is positive. Figure 7 shows a positive offset error.

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