HCS08 datasheet(118/198 Pages) NXP

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Analog-to-Digital Converter (S08ADC10V1)

MC9S08QD4 Series MCU Data Sheet, Rev. 6

118

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 1LSB, 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.

8.6.2.4

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 1LSB, is:

1LSB = (VREFH - VREFL) / 2

Eqn. 8-2

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

± 1/2LSB in 8- or 10-bit mode. As a consequence, however, the code width of the first ($000)

conversion is only 1/2LSB and the code width of the last ($FF or $3FF) is 1.5LSB.

8.6.2.5

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 (EZS) (sometimes called offset) — This error is defined as the difference between

the actual code width of the first conversion and the ideal code width (1/2LSB). Note, if the first

conversion is $001, then the difference between the actual $001 code width and its ideal (1LSB) is

used.

•

Full-scale error (EFS) — This error is defined as the difference between the actual code width of

the last conversion and the ideal code width (1.5LSB). Note, if the last conversion is $3FE, then the

difference between the actual $3FE code width and its ideal (1LSB) 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.

8.6.2.6

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

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