Datasheet

286
2549O–AVR–05/12
ATmega640/1280/1281/2560/2561
26.6.3 Offset Compensation Schemes
The stage has a built-in offset cancellation circuitry that nulls the offset of differential measure-
ments as much as possible. The remaining offset in the analog path can be measured directly by
selecting the same channel for both differential inputs. This offset residue can be then sub-
tracted in software from the measurement results. Using this kind of software based offset
correction, offset on any channel can be reduced below one LSB.
26.6.4 ADC Accuracy Definitions
An n-bit single-ended ADC converts a voltage linearly between GND and V
REF
in 2
n
steps
(LSBs). The lowest code is read as 0, and the highest code is read as 2
n
-1.
Several parameters describe the deviation from the ideal behavior:
• Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition
(at 0.5 LSB). Ideal value: 0 LSB.
Figure 26-11. Offset Error
• Gain Error: After adjusting for offset, the Gain Error is found as the deviation of the last
transition (0x3FE to 0x3FF) compared to the ideal transition (at 1.5 LSB below maximum).
Ideal value: 0 LSB.
Figure 26-12. Gain Error
Output Code
V
REF
Input Voltage
Ideal ADC
Actual ADC
Offset
Error
Output Code
V
REF
Input Voltage
Ideal ADC
Actual ADC
Gain
Error