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16.7.6
Notes on Noise Countermeasures
A protection circuit connected to prevent damage due to an abnormal voltage such as an excessive
surge at the analog input pins (AN0 to AN7, AN12, AN13) should be connected between AVcc
and AVss as shown in figure 16.7. Also, the bypass capacitors connected to AVcc and the filter
capacitor connected to AN0 to AN15 must be connected to AVss.
If a filter capacitor is connected, the input currents at the analog input pins (AN0 to AN7, AN12,
AN13) 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
), an error will arise in the analog input pin voltage. Careful consideration is therefore required
in
when deciding the circuit constants.
1
1
*
*
Notes:
Values are reference values.
*1
10 µF
*2 R
: Input impedance
in
Figure 16.7 Example of Analog Input Protection Circuit
Rev. 2.00, 05/03, page 634 of 820
2
R
*
100
in
0.1 µF
0.01 µF
AV
CC
Vref
AN0 to AN7, AN12, AN13
AV
SS
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