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AN3029 Analog-to-digital converter (ADC)
41
3 Analog-to-digital converter (ADC)
This section does not apply to STM8L101xx devices.
3.1 Analog power
For some packages, the ADC unit has an independent, analog supply voltage, isolated on
input pin V
DDA
, which allows the ADC to accept a very clean voltage source. This analog
voltage, V
DDA
, should be identical to the digital voltage supply on pin V
DD
. To filter some
noise, a ferrite bead can be added between V
DD
and V
DDA
. This ferrite bead should be
chosen according to the frequencies to be filtered.
Some packages also provide a separate external analog reference voltage input for the
ADC unit on the V
REF+
pin. This gives better accuracy on low voltage input as follows:
• The V
REF+
pin can be connected to the V
DDA
external power supply. If a separate,
external reference voltage is applied on V
REF+
, a 100 nF and a 1 µF capacitor must be
connected on this pin. To compensate for peak consumption on V
REF
, the 1 µF
capacitor may be increased to 10 µF when the sampling speed is low. In all cases,
V
REF+
must be kept between 2.4 V and V
DDA
. If V
DDA
is below 2.4, V
REF+
must be
equal to V
DDA
. This input is internally bonded to V
DDA
in the devices that have no
internal V
REF+
pin.
• V
REF-
(input, analog reference negative): The lower/negative reference voltage is
internally bonded to V
SSA
.
3.2 Analog input
The devices include up to 28 analog input channels (including four fast channels), each
multiplexed with an I/O, which are converted by the ADC one at a time.
The external input impedance (R
AIN
) max value is 50 kΩ. The four fast channels can convert
at maximum speed (1 MHz) if
R
AIN
is less than 0.5 kΩ.
Please refer to Figure 3.
Figure 3. Analog input interface
Please refer to the applicable datasheets and reference manuals for more details.
MS32530V1
Sample and hold
ADC converter
12-bit
converter
RADC
CADC
VT
0.6 V
VT
0.6 V
VDD
AINx
RAIN
Cparasitic
IL
VAIN
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