ST STM32 Application Note page 44

How to get the best ADC accuracy
An extra large C
However, increasing C
the "external" timing constant R
The formulas below show how to choose the optimal C
correlation with sample time. Signal bandwidth is characterized by an "external" timing
constant, so optimal solution is to charge C
( R ⋅ C
in
( ⋅
R C
in ext
1 In 1
– =
1
–
e 1
=
After simplification we obtain the final formula for optimal C
C
C
= ----------------------- -
ext
and the corresponding waiting time between conversions:
Practically the firmware must not program the ADC in continuous mode but only in single
mode and must ensure that there will be a time gap between conversions with duration
equal to t
together with the hardware change (adding an external capacitor C
Without implementation of t
after the first one) the external capacitor C
capacitor. After a lot of cycles the voltage on C
previously shown in
A practical example of implementation for STM32L1 ADC is shown below:
C = 16 pF
sh
R = 150 kΩ
in
44/49
(C >>
C
ext ext
sh
limits the frequency bandwidth of measurement signal (increasing
ext
) t
=
ext C
) ( ⋅ ) ⋅
R C In 1
= –
in ext
C U
sh max
---------- - ------------- -
–
C U
ext lsb
C U
sh max
– ---------- - ------------- -
C U
ext lsb
U
max
------------- -
sh
U U
lsb
≈ 1 58 , ⋅ C
------------- -
sh
1
–
1 e
–
t ≈ ( R
–
C in
. This adding of waiting time is the software change which must be applied
C
waiting time in software (for instance, running a conversion just
C
Figure 36).
..... ADC property
..... signal source property
U
max
) enables sampling more often.
⋅
------------- -
U
lsb
. C ).
in ext
during t
ext
C U
sh max
– ---------- - ------------- -
C U
ext lsb
max
U
lsb
1
⋅ C ) ⋅ In 1
– --------------
ext
,
1 58
will be cyclically charged from the C
ext
will reach a quite high error value (as
ext
DocID15067 Rev 3
value: signal bandwidth in
ext
:
C
:
ext
≈ ( R ⋅ C )
in ext
).
ext
AN2834
sh