Datasheet

Section 16 A/D Converter
Rev. 3.00 Mar 17, 2006 page 761 of 926
REJ09B0283-0300
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 and AN12 to AN15) 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 AN7 and AN12 to AN15 must be connected to AVss.
If a filter capacitor is connected, the input currents at the analog input pins (AN0 to AN7 and
AN12 to AN15) 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
in
), an error will arise in the analog input pin voltage. Careful consideration is therefore required
when deciding the circuit constants.
AV CC
*
1
*
1
Vref
AN0 to AN7, AN12 to AN15
AV SS
Notes: Values are reference values.
1.
2. R
in
: Input impedance
R
in
*
2
100Ω
0.1 µF
0.01 µF10 µF
Figure 16.7 Example of Analog Input Protection Circuit