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22.7.6
Notes on Noise Countermeasures
A protection circuit connected to prevent damage of the analog input pins (AN0 to AN11) and
analog reference voltage pin (AVref) due to an abnormal voltage such as an excessive surge
should be connected between AVcc and AVss, as shown in figure 22.6. Also, the bypass
capacitors connected to AVcc and AVref, and the filter capacitors connected to AN0 to AN11
must be connected to AVss.
If a filter capacitor is connected, the input currents at the analog input pins (AN0 to AN11) are
averaged, and so an error may arise. Also, when A/D conversion is performed frequently, as in
scan mode, if the current charged and discharged by the capacitance of the sample-and-hold circuit
in the A/D converter exceeds the current input via the input impedance (R
the analog input pin voltage. Careful consideration is therefore required when deciding the circuit
constants.
1
*
Notes:
Figure 22.6 Example of Analog Input Protection Circuit
Table 22.7 Analog Pin Specifications
Item
Analog input capacitance
Permissible signal-source
impedance
2
R
*
100 Ω
in
1
*
0.1 µF
Values are reference values.
1.
10 µF
0.01 µF
2. R
: Input impedance
in
Min.
in
AV
CC
AV
ref
AN0 to AN11
AV
SS
Max.
20
5
Rev. 1.00 May 09, 2008 Page 709 of 954
Section 22 A/D Converter
), an error will arise in
Unit
pF
kΩ
REJ09B0462-0100
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