AN2438 Freescale Semiconductor / Motorola, AN2438 Datasheet - Page 6

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

AN2438/D
Offset and Gain Error
6
Zero-Scale Errors and Full-Scale Errors can be used to calculate Offset and
Gain Errors. These terms are used to define the performance on many
industry-standard ADC’s but the definitions used vary and can be misleading
or inconsistent.
Consistency with previous Motorola documents can be achieved by replacing
references to Offset Error with Zero-Scale Error and Gain Error with the
difference between of Zero Scale Errors and Full-Scale Errors.
Freescale Semiconductor, Inc.
•
•
•
•
•
For More Information On This Product,
Offset Error (E
difference between the actual and ideal first transition voltages. This is
the same definition as Zero-Scale Error. The term offset; however,
implies that all conversions are off by an equal amount. In the case of a
strong non-linearity near the Zero-Scale Value, this definition may be
misleading, and the less ambiguous Zero-Scale Error term is preferable.
Best-Fit Offset is the difference between the Best-Fit Straight-Line
Transfer Function and the Ideal Straight-Line Transfer Function at
V
((V
impossible to measure in the application and is therefore only a
laboratory curiosity. Since this yields optimistic results and is not
measurable in the application, it can be misleading and will not be used.
Full-Scale Offset is the difference between the actual and ideal last
transition voltages. This is the same definition as Full-Scale Error, and is
misleading for the same reason that Offset is misleading with respect to
Zero-Scale Error.
Gain Error (E
of the Actual and the Ideal Straight-Line Transfer Functions. The error is
not measured as a slope but rather as the difference in the total available
input range from the first to the last conversions between the Ideal and
Adjusted Straight-Line Transfer Functions. It is can also be expressed
by:
Gain Error is not directly measurable and the term has been
inconsistently defined in the literature. Additionally, if there are strong
non-linearities at the endpoints, this definition of Gain Error may be
misleading, so the less ambiguous Full-Scale Error term is preferred
provided a simple gain calculation (above) is possible.
Best-Fit Gain Error is the difference in the slope of the Best-Fit and
Ideal Straight-Line Transfer Functions. The error is not measured as a
slope but rather as the difference in the total available input range from
the first to the last conversions between the Ideal and Best-Fit Straight-
Line Transfer Functions. Since the Best-Fit Straight-Line Transfer
Function will result in an optimistic Gain Error and is virtually impossible
to measure in the application, it can be misleading and will not be used.
REFL
REFH
ADC Definitions and Specifications
E
G
. Some definitions define the offset point at the center-conversion
Go to: www.freescale.com
= E
– V
ZS
REFL
G
– E
O
) or Adjusted Gain Error is the difference in the slope
) /2) instead of at V
), Adjusted Offset, or Zero-Scale Offset is the
FS
REFL
. This offset is virtually
MOTOROLA