To measure the offset error, increase the input voltage from GND until the first transition in the output value occurs. Calculate the difference between the input voltage for which the perfect ADC would have shown the same transition and the input voltage corresponding to the actual transition. This difference, converted to LSB, equals the offset error.
In Figure A, the first transition occurs at 1 LSB. The transition is from 2 to 3, which equals an input voltage of 2½ LSB for the perfect ADC. The difference is +1½ LSB, which equals the offset error. The double-headed arrows show the differences.
The same procedure applies to Figure B. The first transition occurs at 2 LSB. The transition is from 0 to 1, which equals an input voltage of ½ LSB for the perfect ADC. The difference is -1½ LSB, which equals the offset error.
To compensate for offset errors when using single-ended channels, subtract the offset error from every measured value. Be aware that offset errors limit the available range for the ADC. A large positive offset error causes the output value to saturate at maximum before the input voltage reaches maximum. A large negative offset error gives output value 0 for the lowest input voltages.