TC1796
Peripheral Units (Vol. 2 of 2)
Analog-to-Digital Converter (ADC)
User’s Manual 25-37 V2.0, 2007-07
ADC, V2.0
25.1.5 Reference Voltages (V
AREF
and V
AGND
)
The digital result of a conversion represents the analog input as a fraction of the
reference (V
AREF
- V
AGND
) in steps of 2
-n
by n-bit resolution:
Result = 2
n
× (V
AIN
- V
AGND
) / (V
AREF
- V
AGND
)(25.5)
The ADC module offers the choice of four selectable reference voltages V
AREF
[0] to
V
AREF
[3]. The reference voltage can independently be selected for each analog channel
via the respective bit field CHCONn.REF. V
AREF
[0] corresponds to the positive reference
voltage V
AREF
and is used for self calibration of the A/D Converter. Therefore, it must be
stable during all conversions, even for those that use another reference voltage.
The reference voltages must fulfill the following specifications:
V
AREF
[3:0] ≤ V
DDM
+ 0.05 V; V
DDM
≤ 3.3 V (25.6)
A conversion with low reference voltage affects the accuracy of the A/D Converter. The
TUE of an A/D Converter that is operated at a reduced positive reference voltage can be
evaluated according to the following equations:
TUE |
A
→ TUE |
B
= K × TUE |
A
, (K ≥ 1) (25.7)
with factor K as:
V
AREF
|
A
→ V
AREF
|
B
= 1/K × V
AREF
|
A
(25.8)
where
• V
AREF
|
A
: minimum positive reference voltage range is specified
for 0 V ≤ V
AREF
≤ V
DDM
+ 0.05 V
• V
AREF
|
B
: positive reference voltage, which is below the specified range
•TUE |
A
: total unadjusted error for reference voltages within the specified range
•TUE |
B
: total unadjusted error for reference voltages below the specified range
Note: All unused analog input pins must be connected to a fixed potential either V
AGND
or V
AREF
[0] to avoid disturbance of active analog inputs.
Note: It is not recommended in general to set V
AREF
below 50% of V
DDM
.
Note: The analog input voltages V
AIN
must be in the range between V
AGND
and the
selected V
AREF
.
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