ST STM32 Application Note

DocID15067 Rev 3 43/49
AN2834 How to get the best ADC accuracy
48
Software change
The side effect mentioned above can be solved by software. The objective is to create a
delay in order to let C
ext
discharge through R
in
(not measure so often) giving enough
“discharge time” between ADC conversions. The “discharge time” (t
C
) is equal to the
transferred charge from C
sh
to C
ext
(charging) and from C
ext
to R
in
(discharging). The
assumption is that C
ext
>> C
sh
.
Q
ch ingarg
Q
sh
C
sh
U
max
⋅==
where:
U
lsb
....... 0.5 LSB voltage level
U
max
..... 4096 LSB voltage level (worst case)
Q
charging
= Q
discharging
Simplification of the above formula gives the final formula for the required waiting time
between conversions:
This final formula shows dependency between the external capacitor C
ext
and the required
waiting time between two conversions if the precision U
lsb
is needed.
From the same formula you can see that the argument in logarithm must be positive and
theref
ore there is a condition for the minimal value of C
ext
:
Choosing a larger C
ext
decreases more the time between conversions (t
C
).
Q
disch ingarg
U
lsb
R
in
-----------
e
t
R
in
C
ext
-------------------–
td
0
t
C
∫
⋅=
C
sh
U
max
⋅
U
lsb
R
in
-----------
e
t
R
in
C
ext
-------------------
td
0
t
C
∫
⋅=
t
C
R
in
C
ext
⋅()– In 1
C
sh
C
ext
-----------
U
max
U
lsb
------------- -
–⋅=
1
C
sh
C
ext
-----------
U
max
U
lsb
------------- -
– 0>
1
C
sh
C
ext
-----------
U
max
U
lsb
------------- -
>
C
ext
C
sh
U
max
U
lsb
------------- -
⋅>

ST STM32 Specifications

General

Series	STM32
Category	Microcontrollers
Core	ARM Cortex-M0, Cortex-M0+, Cortex-M3, Cortex-M4, Cortex-M7, Cortex-M33
Clock Speed	Up to 480 MHz
GPIO Pins	Up to 144
Communication Interfaces	I2C, SPI, USART, USB, CAN, Ethernet
Package Types	LQFP, BGA, WLCSP
ADC Resolution	12-bit, 16-bit (varies by series)
DAC Resolution	12-bit (varies by series)
Operating Temperature	-40°C to +85°C or +105°C