AN2438 Freescale Semiconductor / Motorola, AN2438 Datasheet - Page 2

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

AN2438/D
ADC Transfer
Curves
Ideal Transfer
Function
2
The ADC converts an input voltage to a corresponding digital code. The curve
describing this behavior is the Actual Transfer Function. The Ideal Transfer
Function represents this behavior assuming the ADC is perfectly linear, or that
a given change in input voltage will create the same change in conversion code
regardless of the input’s initial level. The Adjusted Transfer Function assumes
this behavior after the errors at the endpoint are accounted for.
The Ideal Straight-Line Transfer Function of an ADC is a straight line from the
minimum input voltage (voltage reference low; V
voltage (voltage reference high; V
quantized (divided into steps) by the number of codes the ADC is capable of
resolving. The input voltage range is divided into steps, each step having the
same width.
This Ideal Code Width (ICW) — also known as 1LSB — is:
Where:
The Ideal Transfer Function is then:
Freescale Semiconductor, Inc.
•
•
•
•
ICW = 1LSB = (V
N is the “width” of the ADC, in our examples, 10 bits.
Code = (V
For More Information On This Product,
approximately relates the error voltage to the observed error in
conversion (code error), and is useful for systemic errors such as
differential non-linearity. A 2.56-V input on an ADC with ± 3 LSB of error
could read between $1FD and $203. This unit is by far the most common
terminology and will be the preferred unit used for error representation.
Percent Full-Scale Value (%FSV) — Percent full-scale voltage is a unit
of voltage equal to 1/100
error clarifies the size of the error relative to the input range, and is useful
for trimmable errors such as offset or gain errors. This unit is difficult to
accurately translate to observed error.
Counts — A count is a unit of voltage equal to an LSB. It is a terminology
unique to specifications of some Motorola ADC’s and may be confusing
to customers when doing cross-vendor comparisons.
Bits — A bit is a unit equal to the log (base2) of the error voltage
normalized to the resolution of the ADC. An error of N bits corresponds
to 2
to extrapolate between integer values.
Decibels (db) — A decibel is a unit equal to 20 times the log (base10)
of the error voltage normalized to the full-scale value
(20*log(err_volts/input_range)). A 2.56 input on an ADC with an error of
50 db will convert between $1FD and $203. This figure is often used in
the communications field and is infrequently used in control or
monitoring applications.
ADC Definitions and Specifications
N
LSB of error. This measure is easily confused with LSB and is hard
IN
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– V
REFL
REFH
) / 1LSBV
– V
REFL
th
IN
of the input range of the ADC. This unit of
REFH
)/2
= (Code*1LSB) + V
N
). The Ideal Transfer Function is then
REFL
) to the maximum input
REFL
MOTOROLA