AN2438 Freescale Semiconductor / Motorola, AN2438 Datasheet - Page 9

• Manufacturer Part Number: AN2438
• Description: ADC Definitions and Specifications
• Manufacturer: Freescale Semiconductor / Motorola

MOTOROLA
The three primary definitions are:
There are some related definitions to Total Unadjusted Error that vary slightly
in definition. These are:
1. INL, Adjusted INL, or Endpoint INL — The current transition voltage is
2. Unadjusted INL — The current transition voltage is compared to the
3. Best-Fit INL — The current transition voltage is compared to the
•
•
•
Freescale Semiconductor, Inc.
For More Information On This Product,
compared to the corresponding transition voltage on the Adjusted
Transfer Function. This is a useful indicator of the best the ADC can do
if the endpoint non-linearities (Zero- and Full-Scale Errors) are
measured and trimmed out.
corresponding transition voltage on the Ideal Transfer Function. This is
a measure of the total error except for Quantization Error.
corresponding transition voltage on the Best-Fit Transfer Function. This
will usually give a balanced positive and negative error across the entire
curve so the results look very optimistic, but since it is difficult to obtain
the Best-Fit Transfer Function in application, it is not a very useful
measure. Unfortunately, this is what many evaluation packages
(hardware and software) measure.
Total Error is the same as Total Unadjusted Error, but the term is
misused in several ADC references and is therefore misleading. The
less ambiguous term Total Unadjusted Error is preferred.
Total Adjusted Error is the difference between the Actual and Adjusted
Straight-Line Transfer Function, accounting for INL plus Quantization
Error. This term is redundant and potentially confusing with respect to
Total Unadjusted Error and will not be used.
Code Error is the error between the ideal code and the current code.
This is the only figure of merit that measures by the quantized output
instead of voltage. The code error is the Total Unadjusted in LSB,
rounded to the nearest integer.
$7
$6
$5
$4
$3
$2
$1
Conversion
E
ADC Definitions and Specifications
Total Unadjusted Error
TUE
Figure 4. Total Unadjusted and Code Error Graphs
= 1.75
Input Voltage in LSB
Go to: www.freescale.com
4
Ideal Straight Line
Transfer Function
5
E
TUE
6
= -1.5
7
V
REFH
$7
$6
$5
$4
$3
$2
$1
Conversion
V
REFL
Code Error
= $1 or $2
1
Input Voltage in LSB
Code Error
2
Integral Non-Linearity (INL)
3
4
Code Error = $2
5
6
Transfer
Function
7
Ideal
V
REFH
AN2438/D
9