
Functional Description
MC68HC908QL4  MC68HC908QL3  MC68HC908QL2 Data Sheet, Rev. 4
Freescale Semiconductor
49
3.3.4.4  Code Width and Quantization Error
The ADC10 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 from one code to 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:
1
LSB
 = (V
REFH
–V
REFL
) / 2
N
Because of this quantization, there is an inherent quantization error. Because the converter performs a 
conversion and then rounds to 8 or 10 bits, 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
.
3.3.4.5  Linearity Errors
The ADC10 may also exhibit non-linearity of several forms. Every effort has been made to reduce these 
errors but the user 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.
3.3.4.6  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 very 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 ±1/2 
LSB
 but will 
increase with noise. 
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 which are never converted for any input value. In 8-bit or 10-bit mode, the 
ADC10 is guaranteed to be monotonic and to have no missing codes.