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There is an inherent quantization error due to the digitization of the result. For 8-bit or
10-bit conversions the code transitions when the voltage is at the midpoint between the
points where the straight line transfer function is exactly represented by the actual
transfer function. Therefore, the quantization error will be ± 1/2 lsb in 8- or 10-bit mode.
As a consequence, however, the code width of the first (0x000) conversion is only 1/2 lsb
and the code width of the last (0xFF or 0x3FF) is 1.5 lsb.
For 12-bit conversions the code transitions only after the full code width is present, so the
quantization error is -1 lsb to 0 lsb and the code width of each step is 1 lsb.
17.7.2.5 Linearity errors
The ADC may also exhibit non-linearity of several forms. Every effort has been made to
reduce these errors but the system must be aware of them because they affect overall
accuracy. These errors are:
• Zero-scale error (E
difference between the actual code width of the first conversion and the ideal code
width (1/2 lsb in 8-bit or 10-bit modes and 1 lsb in 12-bit mode). If the first
conversion is 0x001, the difference between the actual 0x001 code width and its ideal
(1 lsb) is used.
• Full-scale error (E
code width of the last conversion and the ideal code width (1.5 lsb in 8-bit or 10-bit
modes and 1LSB in 12-bit mode). If the last conversion is 0x3FE, the difference
between the actual 0x3FE code width and its ideal (1 lsb) is used.
• Differential non-linearity (DNL) — This error is defined as the worst-case difference
between the actual code width and the ideal code width for all conversions.
• Integral non-linearity (INL) — This error is defined as the highest-value that the
absolute value of the running sum of DNL achieves. More simply, this is the worst-
case difference of the actual transition voltage to a given code and its corresponding
ideal transition voltage, for all codes.
• Total unadjusted error (TUE) — This error is defined as the difference between the
actual transfer function and the ideal straight-line transfer function and includes all
forms of error.
NXP Semiconductors
) (sometimes called offset) — This error is defined as the
ZS
) — This error is defined as the difference between the actual
FS
MC9S08SU16 Reference Manual, Rev. 5, 4/2017
Chapter 17 Analog-to-digital converter (ADC)
287
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