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CHAPTER 12 A/D CONVERTER
User
’
s Manual U14701EJ3V0UD
(4) Noise countermeasures
To maintain the 10-bit resolution, attention must be paid to noise input to the AV
REF0
and ANI0 to ANI9 pins.
Because the effect increases in proportion to the output impedance of the analog input source, it is recommended
that a capacitor be connected externally as shown in Figure 12-16 to reduce noise.
Figure 12-16. Analog Input Pin Connection
Reference
voltage
input
C = 100 to 1,000 pF
If there is a possibility that noise equal to or higher than AV
REF0
or
equal to or lower than AV
SS0
may enter, clamp with a diode with a
small V
F
value (0.3 V or lower).
AV
REF0
V
DD1
AV
SS0
V
SS0
ANI0 to ANI9
(5) ANI0 to ANI9
The analog input pins (ANI0 to ANI9) also function as port pins.
When A/D conversion is performed with any of pins ANI0 to ANI9 selected, do not execute an input instruction
to port 1 while conversion is in progress, as this may reduce the conversion resolution.
Also, if digital pulses are applied to other analog input pins during A/D conversion, the expected
A/D conversion value may not be obtainable due to coupling noise. Therefore, avoid applying pulses to other
analog input pins during A/D conversion.
(6) AV
REF0
pin input impedance
A series resistor string is connected between the AV
REF0
pin and the AV
SS0
pin.
Therefore, when the output impedance of the reference voltage is too high, it seems as if the AV
REF0
pin and
the series resistor string are connected in series. This may cause a greater reference voltage error.