Datasheet

Chapter 10 Analog-to-Digital Converter (S08ADC12V1)
MC9S08DN60 Series Data Sheet, Rev 3
192 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 (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 1
LSB 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.
• 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.