Datasheet

Section 16 A/D Converter (8 Analog Input Channel Version)
Rev.6.00 Sep. 27, 2007 Page 718 of 1268
REJ09B0220-0600
16.6 Usage Notes
The following points should be noted when using the A/D converter.
Setting Range of Analog Power Supply and Other Pins
1. Analog input voltage range
The voltage applied to analog input pins ANn during A/D conversion should be in the range
AV
SS
≤ ANn ≤ V
ref
.
2. Relation between AV
CC
, AV
SS
and V
CC
, V
SS
As the relationship between AV
CC
, AV
SS
and V
CC
, V
SS
, set AV
SS
= V
SS
. If the A/D converter is
not used, the AV
CC
and AV
SS
pins must not be left open.
3. V
ref
input range
The analog reference voltage input at the V
ref
pin should be set in the range V
ref
≤ AV
CC
. The
V
ref
pin should be set as V
ref
= V
CC
when the A/D converter is not used. Do not leave the V
ref
pin open.
If conditions 1, 2, and 3 above are not met, the reliability of the device may be adversely affected.
Notes on Board Design: In board design, digital circuitry and analog circuitry should be as
mutually isolated as possible, and layout in which digital circuit signal lines and analog circuit
signal lines cross or are in close proximity should be avoided as far as possible. Failure to do so
may result in incorrect operation of the analog circuitry due to inductance, adversely affecting A/D
conversion values.
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
16.7.
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 16.7, 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