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TC1784
Analog to Digital Converter (ADC)
User´s Manual
23-14
V1.1, 2011-05
ADC, V1.3
23.1.9
Transfer Characteristics and Error Definitions
The ideal transfer characteristic of the ADC translates a continuous analog input voltage
into a discrete digital value out of a result range of 2
n
steps for n bit resolution over a
measurement range between 0 and a reference voltage. Each digital value in the
available result range (from 0 to 2
n
-1) represents an input voltage range that is defined
by the reference voltage divided by 2
n
. This range (called quantization step) represents
the smallest granularity (called LSB
n
) that can be handled by the ADC. Due to the
discrete character of the digital result, each ADC conversion result has a system-
inherent quantization uncertainty of ± 0.5 LSB
n
. According to the ideal transfer
characteristics, the first digital transition (between the digital values 0 and 1) takes place
when the analog input reaches 0.5 LSB
n
.
An analog input voltage above the reference voltage leads to a saturation of the digital
result at 2
n
-1.
Deviations of the conversion result from the ideal transfer characteristics can appear:
•
An
offset error
is the deviation from the ideal transfer characteristics for an input
voltage close to 0. It describes the difference between 0.5 LSB
n
and the input voltage
where the first digital transition (between the values of 0 and 1) occurs.
•
A
gain error
is the deviation from the ideal transfer characteristics for an input
voltage close to the reference voltage. It describes the difference between the
reference voltage and the input voltage where the last digital transition (between the
values of 2
n
-2 and 2
n
-1) occurs.
•
A
differential non-linearity error
(DNL) describes the variations in the analog input
voltage between two adjacent digital conversion results, over the full measurement
range. If each step between the digital conversion results x and x+1 is exactly 1 LSB
n
,
the DNL value is zero. If the DNL value is lower than 1 LSB
n
, the possibility of missing
codes is excluded. A missing code occurs if not all values of the possible conversion
result range can be reached.
•
An
integral non-linearity error
(INL) describes the maximum difference between
the transfer characteristics between the first and the last point of the measurement
range and the real transfer characteristics (without quantization uncertainty, offset
and gain errors).
•
The
total unadjusted error
(TUE) describes the maximum deviation between a real
conversion result and the ideal transfer characteristics over a given measurement
range. Since some of these errors noted above can compensate each other, the TUE
value generally is much less than the sum of the individual errors.
The TUE also covers production process variations and internal noise effects (if
switching noise is generated by the system, this generally leads to an increased TUE
value).
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