AN2438 Freescale Semiconductor / Motorola, AN2438 Datasheet - Page 4

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

AN2438/D
Adjusted Transfer Function
Best-Fit Transfer Function
4
The Adjusted Transfer Function is then quantized in the same method as the
Ideal Transfer Function. The Adjusted Code Width is therefore:
The Adjusted Transfer Function is then:
Transfer Function are not useful to the user. Unfortunately, many automatic
evaluation packages (software and hardware) assume this type of curve.
$7
$6
$5
$4
$3
$2
$1
Conversion
Best-Fit E
½LSB Comp. Best-Fit TF
Freescale Semiconductor, Inc.
ACW = [(V
Code = (V
$7
$6
$5
$4
$3
$2
$1
Conversion
Zero-Scale Error
½LSB Comp. Adjusted TF
Function (Double)
For More Information On This Product,
Actual Transfer
Input Voltage in LSB
Code Width
Code Width
Input Voltage in LSB
Best-Fit
Adjusted
ZS
ADC Definitions and Specifications
IN
Go to: www.freescale.com
REFH
4
– V
Best-Fit Straight
Function (Solid)
(-) Full-Scale Error
(+) Best-Fit E
4
Line Transfer
5
REFL
+ E
Function (Dashed)
Adjusted Transfer
Adjusted Straight-
Function (Solid)
5
Line Transfer
6
FS
(Dashed)
Function
Transfer
Best-Fit
6
7
– E
) – (V
V
7
REFH
FS
ZS
V
REFH
) / ACW
REFL
Some ADC’s exhibit low Zero- and Full-Scale
Errors but still have significant non-linearities.
In cases where these linearities tend to be in
one direction (for example, a significantly
“bowed” function) the best application results
may be obtained if the errors are compared to
a Best-Fit Transfer Function. The Best-Fit
Straight-Line Transfer Function is the line from
which the average deviation of all conversions
is minimum. Computing this function requires
that the entire Actual Transfer Function be
recorded, which is impractical in most
applications. Therefore, any performance
parameters calculated against the Best-Fit
+ E
Most ADC’s exhibit some non-linearity at
the endpoints due to the difficulty in
measuring a signal that is identical to the
reference. If these errors are accounted
for, a more accurate portrayal of the ADC
behavior in application is possible. For
this reason, the Adjusted Straight-Line
Transfer Function is drawn between the
minimum input voltage plus the Zero-
Scale Error (V
maximum input voltage plus the Full-
Scale Error (V
ZS
V
)] / 2
IN
= (Code*ACW) + V
N
REFH
REFL
+ E
+ E
ZS
FS
) to the
).
REFL
MOTOROLA
+ E
ZS