ST STM32

How to get the best ADC accuracy AN2834
30/49 DocID15067 Rev 3
3.2.7 Source frequency condition vs. source and parasitic capacitors
The external capacitance will not allow the analog input voltage to be exactly the same as
V
AIN
if the capacitor is not fully charged by the analog source.
If the analog input signal changes, then the analog signal frequency (F
AIN
) should be such
that the time period of this analog signal is at least: 10 × R
AIN
× (C
AIN
+ C
p
).
T
AIN
= analog signal time period = 1/F
AIN
.
We have:
T
AIN
10 R
AIN
C
AIN
C
P
+()××≥
Therefore:
F
AIN
1
10 R
AIN
C
AIN
C
P
+()××
-----------------------------------------------------------------------
≤
For example:
For R
AIN
= 25 kΩ, C
AIN
= 7 pF, C
p
= 3 pF, this gives:
F
AINmax
1
10 25 10
3
× 73+()10
12–
×××
--------------------------------------------------------------------------------------=
Thus, the maximum frequency of the source will be:
F
AINmax
400 kHz=
.
So for the above defined source characteristics (capac
itance and resistance), the frequency
of the source must not exceed 400 kHz, otherwise the ADC conversion result will be not
ac
curate.
Figure 26. Recommended values for R
AIN
and C
AIN
vs. source frequency F
AIN
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