12-bit Analog-to-Digital Converter (S08ADCV1)
MC9S08QE128 MCU Series Reference Manual, Rev. 2
200
Freescale Semiconductor
•
Average the result by converting the analog input many times in succession and dividing the sum
of the results. Four samples are required to eliminate the effect of a 1
LSB
, one-time error.
•
Reduce the effect of synchronous noise by operating off the asynchronous clock (ADACK) and
averaging. Noise that is synchronous to ADCK cannot be averaged out.
10.6.2.4
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 1
LSB
, is:
1
LSB
= (V
REFH
- V
REFL
) / 2
N
Eqn. 10-2
There is an inherent quantization error due to the digitization of the result. For 8-bit or 10-bit conversions
the code will transition 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 ($000)
conversion is only 1/2
LSB
and the code width of the last ($FF or $3FF) 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
.
10.6.2.5
Linearity Errors
The ADC may also exhibit non-linearity of several forms. Every effort has been made to reduce these
errors but the system should 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). Note, if the first conversion is $001, then the difference between
the actual $001 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 1
LSB
in 12-bit
mode). Note, if the last conversion is $3FE, then the difference between the actual $3FE 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 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 therefore includes all forms of error.
Содержание MC9S08QE128
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Страница 49: ...Chapter 3 Modes of Operation MC9S08QE128 MCU Series Reference Manual Rev 2 50 Freescale Semiconductor ...
Страница 138: ...Chapter 6 Parallel Input Output Control MC9S08QE128 MCU Series Reference Manual Rev 2 138 Freescale Semiconductor ...
Страница 144: ...Chapter 7 Keyboard Interrupt S08KBIV2 MC9S08QE128 MCU Series Reference Manual Rev 2 144 Freescale Semiconductor ...
Страница 166: ...Chapter 8 Central Processor Unit S08CPUV4 MC9S08QE128 MCU Series Reference Manual Rev 2 166 Freescale Semiconductor ...
Страница 174: ...MC9S08QE128 MCU Series Reference Manual Rev 2 174 Freescale Semiconductor Analog Comparator S08ACMPV3 ...
Страница 202: ...12 bit Analog to Digital Converter S08ADCV1 MC9S08QE128 MCU Series Reference Manual Rev 2 202 Freescale Semiconductor ...
Страница 282: ...Serial Peripheral Interface S08SPIV3 MC9S08QE128 MCU Series Reference Manual Rev 2 282 Freescale Semiconductor ...
Страница 306: ...Timer PWM Module S08TPMV3 MC9S08QE128 MCU Series Reference Manual Rev 2 306 Freescale Semiconductor ...
Страница 320: ...Development Support MC9S08QE128 MCU Series Reference Manual Rev 2 320 Freescale Semiconductor ...