MC9S08DZ96F2CLL datasheet(232/458 Pages) FREESCALE

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Chapter 10 Analog-to-Digital Converter (S08ADC12V1)

MC9S08DZ128 Series Data Sheet, Rev. 1

232

Freescale Semiconductor

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 (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 (1/2 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 (EFS) — 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 (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 includes all forms of error.

10.6.2.6

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

converter yields the lower code (and vice-versa). However, even small amounts of system noise can cause

the converter to be indeterminate (between two codes) for a range of input voltages around the transition

voltage. This range is normally around 1/2lsb in 8-bit or 10-bit mode, or around 2 lsb in 12-bit mode, and

increases with noise.

This error may be reduced by repeatedly sampling the input and averaging the result. Additionally the

techniques discussed in Section 10.6.2.3 reduces this error.

Non-monotonicity is deﬁned as when, except for code jitter, the converter converts to a lower code for a

higher input voltage. Missing codes are those values never converted for any input value.

In 8-bit or 10-bit mode, the ADC is guaranteed to be monotonic and have no missing codes.

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