Analog-to-Digital Converter (ADC) absolute error (absolute accuracy) is the total uncompensated error and includes quantization error, offset error, gain error and non-linearity. It's is the amount of deviation from the ideal ADC transfer function without compensating for gain or offset errors. The absolute error is also called the total unadjusted error. This error specification gives the designer details of the worst-case A/D converter performance without any form of error compensation.
Ideally, the absolute error is 0.5 LSB which is due to the quantization error. Gain and offset errors are more significant contributors of absolute error. The absolute error represents a reduction in the ADC range. Engineers should consider keep some margins against the minimum and maximum input values to avoid the absolute error impact