Source Frequency Condition Vs. Source And Parasitic Capacitors; Ain And Cain Vs. Source Frequency Fain - ST STM32 Application Note

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