Analog-to-digital converter (ADC) RM0444
378/1390 RM0444 Rev 5
Calculating the actual V
REF+
voltage using the internal reference voltage
V
REF+
voltage may be subject to variation or not precisely known. The embedded internal
reference voltage (V
REFINT
) and its calibration data acquired by the ADC during the
manufacturing process at V
REF+_charac
can be used to evaluate the actual V
REF+
voltage
level.
The following formula gives the actual V
REF+
voltage supplying the device:
Where:
• V
REF+_Charac
is the value of V
REF+
voltage characterized at V
REFINT
during the
manufacturing process. It is specified in the device datasheet.
• VREFINT_CAL is the VREFINT calibration value
• VREFINT_DATA is the actual VREFINT output value converted by ADC
Converting a supply-relative ADC measurement to an absolute voltage value
The ADC is designed to deliver a digital value corresponding to the ratio between the analog
power supply and the voltage applied on the converted channel. For most application use
cases, it is necessary to convert this ratio into a voltage independent of V
REF+
. For
applications where V
REF+
is known and ADC converted values are right-aligned you can
use the following formula to get this absolute value:
For applications where V
REF+
value is not known, you must use the internal voltage
reference and V
REF+
can be replaced by the expression provided in Section : Calculating
the actual V
REF+
voltage using the internal reference voltage, resulting in the following
formula:
Where:
• V
REF+_Charac
is the value of V
REF+
voltage characterized at V
REFINT
during the
manufacturing process. It is specified in the device datasheet.
• VREFINT_CAL is the VREFINT calibration value
• ADC_DATA
x
is the value measured by the ADC on channelx (right-aligned)
• VREFINT_DATA is the actual VREFINT output value converted by the ADC
• full_SCALE is the maximum digital value of the ADC output. For example with 12-bit
resolution, it is 2
12
- 1 = 4095 or with 8-bit resolution, 2
8
- 1 = 255.
Note: If ADC measurements are done using an output format other than 12 bit right-aligned, all the
parameters must first be converted to a compatible format before the calculation is done.
15.10 Battery voltage monitoring
The VBATEN bit in the ADC_CCR register allows the application to measure the backup
battery voltage on the VBAT pin. As the V
BAT
voltage could be higher than V
REF+
, to ensure
V
REF+
V
REF _Charac+
VREFINT_CAL VREFINT_DATA⁄×=
V
CHANNELx
V
REF+
FULL_SCALE
-------------------------------------
ADC_DATA
x
×=
V
CHANNELx
V
REF+_Charac
VREFINT_CAL ADC_DATA
x
××
VREFINT_DATA FULL_SCALE×
-------------------------------------------------------------------------------------------------------------------------=
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