15.6.2.4 Code width and quantization error
The ADC quantizes the ideal straight-line transfer function into 4096 steps (in 12-bit
mode). Each step ideally has the same height (1 code) and width. The width is defined as
the delta between the transition points to one code and the next. The ideal code width for
an N bit converter (in this case N can be 8, 10 or 12), defined as 1LSB, is:
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.
15.6.2.5 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
ZS
) (sometimes called offset) — This error is defined as the
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
FS
) — This error is defined as the difference between the actual
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.
Chapter 15 Analog-to-digital converter (ADC)
MC9S08PA4 Reference Manual, Rev. 5, 08/2017
NXP Semiconductors
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Summary of Contents for MC9S08PA4
Page 1: ...MC9S08PA4 Reference Manual Supports MC9S08PA4 Document Number MC9S08PA4RM Rev 5 08 2017 ...
Page 2: ...MC9S08PA4 Reference Manual Rev 5 08 2017 2 NXP Semiconductors ...
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Page 28: ...System clock distribution MC9S08PA4 Reference Manual Rev 5 08 2017 28 NXP Semiconductors ...
Page 150: ...Port data registers MC9S08PA4 Reference Manual Rev 5 08 2017 150 NXP Semiconductors ...
Page 196: ...Human machine interfaces HMI MC9S08PA4 Reference Manual Rev 5 08 2017 196 NXP Semiconductors ...
Page 224: ...Instruction Set Summary MC9S08PA4 Reference Manual Rev 5 08 2017 224 NXP Semiconductors ...
Page 232: ...Functional Description MC9S08PA4 Reference Manual Rev 5 08 2017 232 NXP Semiconductors ...
Page 258: ...FTM Interrupts MC9S08PA4 Reference Manual Rev 5 08 2017 258 NXP Semiconductors ...
Page 294: ...Functional description MC9S08PA4 Reference Manual Rev 5 08 2017 294 NXP Semiconductors ...
Page 398: ...Resets MC9S08PA4 Reference Manual Rev 5 08 2017 398 NXP Semiconductors ...
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