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Also, digital circuitry must be isolated from the analog input signals (AN0 to AN15), analog
reference power supply (Vref), and analog power supply (AVCC) by the analog ground (AVSS).
Also, the analog ground (AVSS) should be connected at one point to a stable digital ground (VSS)
on the board.
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) and analog
reference power supply (Vref ) should be connected between AVCC and AVSS as shown in
figure 19-7.
Also, the bypass capacitors connected to AVCC and Vref and the filter capacitor connected to
AN0 to AN15 must be connected to AVSS.
If a filter capacitor is connected as shown in figure 19-7, the input currents at the analog input 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
), 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 19-7 Example of Analog Input Protection Circuit
100 Ω
2
R
*
in
0.1 µF
0.01 µF
AVCC
Vref
AN0 to AN15
AVSS
763
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