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AN3137 Fiches technique(PDF) 31 Page - STMicroelectronics

No de pièce AN3137
Description  Analog-to-digital converter on STM8L and STM8AL devices
PDF  42 Pages
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Fabricant  STMICROELECTRONICS [STMicroelectronics]
Site Internet  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN3137 Fiches technique(HTML) 31 Page - STMicroelectronics

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AN3137
Methods for precision improvement
Doc ID 16983 Rev 3
31/42
where Ulsb ..... 0.5 lsb voltage level
Qcharging = Qdischarging
After simplification, we obtain the final formula for the required waiting time between
conversions:
This final formula links the external capacitor Cext and the required waiting time between two
conversions if the precision Ulsb is needed.
From the same formula you can see that the argument in logarithm must be positive and
therefore there is a condition for the minimal value of Cext:
Choosing a larger Cext decreases more the time between conversions (tC).
An extra large Cext (Cext>>Csh
U
max
U
lsb
--------------
⋅
) enables sampling more often.
However, increasing Cext limits the frequency bandwidth of measurement signal (increasing
the “external” timing constant Rin . Cext).
The formulas below show how to choose the optimal Cext value: signal bandwidth in
correlation with sample time. Signal bandwidth is characterized by an “external” timing
constant, so:
C
sh
U
max
⋅
U
lsb
R
in
-----------
e
t
R
in Cext
-------------------
t
d
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
--------------
⋅
>
R
in
C
ext
⋅
()
t
C
=
R
in
C
ext
⋅
()
R
in
C
ext
⋅
()
–
In 1
C
sh
C
ext
-----------
U
max
U
lsb
--------------
–
⋅
=



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