
520
Also, digital circuitry must be isolated from the analog input signals (AN0 to AN7), analog
reference power supply (V
ref
), and analog power supply (AV
CC
) by the analog ground (AV
SS
).
Also, the analog ground (AV
SS
) should be connected at one point to a stable digital ground (V
SS
)
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 AN7) and analog
reference power supply (V
ref
) should be connected between AV
CC
and AV
SS
as shown in figure
14.7.
Also, the bypass capacitors connected to AV
CC
and V
ref
and the filter capacitor connected to AN0
to AN7 must be connected to AV
SS
.
If a filter capacitor is connected as shown in figure 14.7, the input currents at the analog input pins
(AN0 to AN7) 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
V
ref
AN0 to AN7
AV
SS
Notes:
Values are reference values.
1.
2. R
in
: Input impedance
R
in
*
2
100
0.1
μ
F
0.01
μ
F
10
μ
F
Figure 14.7 Example of Analog Input Protection Circuit