By Len Staller
Embedded Systems Design
(02/24/05, 05:24:00 PM EST)
Offset error, full-scale error
The ideal transfer function line will intersect the origin of the plot. The first code boundary will occur at 1 LSB as shown in Figure 1. You can observe offset error as a shifting of the entire transfer function left or right along the input voltage axis, as shown in Figure 3.
Figure 3: Offset error
An error of - 1/2 LSB is intentionally introduced into some ADCs but is still included in the specification in the data sheet. Thus, the offset-error specification posted in the data sheet includes 1/2 LSB of offset by design. This is done to shift the potential quantization error in a measurement from 0 to 1 LSB to - 1/2 to +1/2 LSB. In this way, the magnitude of quantization error is intended to be < style="text-align: center;">
Figure 4: Quantization error vs. output code
Figure 5: Full-scale error
Full-scale error is the difference between the ideal code transition to the highest output code and the actual transition to the output code when the offset error is zero. This is observed as a change in slope of the transfer function line as shown in Figure 5. A similar specification, gain error, also describes the non-ideal slope of the transfer function as well as what the highest code transition would be without the offset error. Full-scale error accounts for both gain and offset deviation from the ideal transfer function. Both full-scale and gain errors are commonly used by ADC manufacturers.
Reference:
- http://www.embended.com/
- http://en.wikipedia.org
