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CHAPTER 13 A/D CONVERTER
User’s Manual U12697EJ3V0UM
(4) Anti-noise measures
Attention must be paid to noise fed to AV
DD
and ANI0 to ANI7 to preserve the 8-bit resolution. The influence of
noise grows proportionally to the output impedance of the analog input source. Therefore, it is recommended to
connect C externally, as shown in Figure 13-15.
Figure 13-15. Handling of Analog Input Pin
(5) ANI0/P10 to ANI7/P17
The analog input pins (ANI0 to ANI7) can also be used as an input port pin (P10 to P17).
Do not execute the input command that corresponds with PORT 1 during conversion if any pin from between ANI0
to ANI7 has been selected and A/D conversion run. This would result in a lowered resolution.
Moreover, if a digital pulse is applied to other analog input pins during A/D conversion, the A/D conversion value
will not be obtained as expected because of coupling noise. Therefore, do not apply a pulse to other analog input
pins during A/D conversion.
(6) Input impedance of AV
DD
pin
For the
µ
PD784225, the AV
DD
pin can also be used as the reference voltage source and a series resistor string
of approximately 46 k
Ω
is connected between the AV
DD
and AV
SS
pins.
Therefore, if the output impedance of the reference voltage source is high, connecting in series a series resistor
string between the AV
DD
and AV
SS
pins will result in a large reference voltage error.
ANI0 to ANI7
AV
DD
V
DD
AV
SS
V
SS1
C = 100 to 1000 pF
If there is the possibility that noise equal to or greater than AV
DD
,
or equal to or smaller than AV
SS
, may enter,
clamp with a diode having a small V
F
(0.3 or less).
V
DD0
V
DD0