AN1636 datasheet(28/42 Pages) STMICROELECTRONICS

28 / 42 page

28/42

UNDERSTANDING AND MINIMISING ADC CONVERSION ERRORS

5.1 OFFSET ERROR

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

The first transition is when the digital output of ADC changes from 0 to 1. In an ideal case we

should get a digital output of 1, when the analog input is between 0.5 LSB to 1.5LSB. The first

transition in an ideal case will be at 0.5 LSB. Offset error is represented as EO.

Example 1:

In a 10-bit ADC, ideally 2.44mV (0.5 LSB = 0.5 * 4.88mV) input should generate a corre-

sponding digital output of 1. But in practice the ADC may still show the reading as 0. If we get

a digital output of 1 from an analog input of 10mV, then:

Offset error = Actual transition - Ideal transition

EO = 10mV - 2.44mV = 7.56 mV

EO = 7.56mV / 4.88 mV = 1.54 LSB

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

offset error will be positive.

Figure 25. Positive offset error representation

When the analog input voltage VIN = VSSA and the ADC generates a non-zero digital output

then the offset error will be negative. This will mean that theoretically (or by Extrapolation) a

negative voltage will generate the first transition. As specified in the datasheet, any voltage

less than VSSA will cause the digital output to be 0.

VIN

Digital

output

(Decimal

steps)

Ideal

transfer curve

EO = Offset error (positive)

Actual

transfer curve

0.5LSB

AN1636 Datasheet(PDF) 28 Page - STMicroelectronics