Section 15 A/D Converter
Rev. 3.00 Mar. 14, 2006 Page 565 of 804
REJ09B0104-0300
15.7.6 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 AN15) should be connected between AVcc0, AVcc1 and
AVss as shown in figure 15.10. Also, the bypass capacitors connected to AVcc0 and AVcc1 and
the filter capacitor connected to pins AN0 to AN15 must be connected to AVss.
If a filter capacitor is connected, the input currents at pins AN0 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
CC0
and AV
CC1
*
1
AN0 to AN15
AV
SS
Notes: Values are reference values.
1.
2. R
in
: Input impedance
R
in
*
2
100
Ω
0.1 µF
0.01 µF
10 µF
Figure 15.10 Example of Analog Input Protection Circuit
Table 15.6 Analog Pin Specifications
Item Min
Max
Unit
Analog input capacitance
20 pF
Permissible signal source impedance
5 k
Ω
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