ST STM32

DocID15067 Rev 3 29/49
AN2834 How to get the best ADC accuracy
48
By combining equations (1), (2) and (3), we obtain final formula:
R
AINmax
T
s
f
ADC
C
ADC
2
N1+
()ln⋅⋅
------------------------------------------------------------------------- R
ADCmax
–=
Example for STM32F1 parameters
f
ADC
= 14 MHz, C
ADC
= 8 pF, R
ADC
max = 1 kΩ and for T
s
= 7.5, the maximum source
resistance allowed for an error equal to 1/2 LSB is:
R
AINmax
7.5
14 10
6
810
12–
2
12 1+
()ln⋅⋅⋅ ⋅
------------------------------------------------------------------------------------------------ 1kΩ–=
That is:
R
AINmax
= 6.4 kΩ
Note: The use of a follower amplifier can reduce the resistance of the source effect because of its
high input impedance and its very low output impedance. It isolates R
AIN
from R
ADC
.
However, the amplifier introduces an offset error that should be taken into account.
In case of longer sampling times and reduced number of ADC clocks, better results can be
obt
ained. It is possible to further increase the allowed external resistance by decreasing the
ADC clock frequency or selecting a lower resolution. Refer to the datasheet of your device
to obtain the exact values of the RC parameters.