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AN2834 データシート(PDF) 39 Page - STMicroelectronics

部品番号 AN2834
部品情報  How to get the best ADC accuracy in STM32Fx Series and STM32L1 Series devices
PDF  45 Pages
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メーカー  STMICROELECTRONICS [STMicroelectronics]
ホームページ  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2834 データシート(HTML) 39 Page - STMicroelectronics

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AN2834
How to get the best ADC accuracy
44
Software change:
The side effect mentioned above can be solved by software. The objective is to create a
delay in order to let Cext discharge through Rin (not measure so often) giving enough
“discharge time” between ADC conversions. The “discharge time” (tC) is equal to the
transferred charge from Csh to Cext (charging) and from Cext to Rin (discharging). The
assumption is that Cext >> Csh.
where:
Ulsb ....... 0.5 lsb voltage level
Umax ..... 4096 lsb voltage level (worst case)
Qcharging = Qdischarging
Simplification of the above formula gives the final formula for the required waiting time
between conversions:
This final formula shows dependency between 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>>
) enables sampling more often.
However, increasing Cext limits the frequency bandwidth of measurement signal (increasing
the “external” timing constant Rin . Cext).
Qch ing
arg
Qsh
Csh Umax
==
Qdisch ing
arg
Ulsb
Rin
-----------
e
t
RinCext
-------------------
–
t
d
0
tC
=
Csh Umax
Ulsb
Rin
-----------
e
t
RinCext
-------------------
t
d
0
tC
=
tC
Rin Cext

–
In 1
Csh
Cext
-----------
Umax
Ulsb
--------------
–
=
1
Csh
Cext
-----------
Umax
Ulsb
--------------
–
0
1
Csh
Cext
-----------
Umax
Ulsb
--------------
Cext Csh
Umax
Ulsb
--------------
Csh
Umax
Ulsb
--------------



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