Datasheet

Chapter 9 Analog-to-Digital Converter (S08ADC10V1)
MC9S08SH8 MCU Series Data Sheet, Rev. 3
146 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.
9.6.2.4 Code Width and Quantization Error
The ADC quantizes the ideal straight-line transfer function into 1024 steps (in 10-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 or 10),
defined as 1
LSB, is:
1LSB = (V
REFH
- V
REFL
) / 2
N
Eqn. 9-2
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 1.5LSB.
9.6.2.5 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). 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). 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.
9.6.2.6 Code Jitter, Non-Monotonicity and Missing Codes
Analog-to-digital converters are susceptible to three special forms of error. These are code jitter,
non-monotonicity, and missing codes.
Code jitter is when, at certain points, a given input voltage converts to one of two values when sampled
repeatedly. Ideally, when the input voltage is infinitesimally smaller than the transition voltage, the