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
.
