161
CHAPTER 13 A/D CONVERTER
(4) Noise countermeasures
To maintain 8-bit resolution, attention must be paid to noise input to pin AV
REF
and pins ANI0 to ANI4. 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 13-10 to reduce noise.
Figure 13-10. Analog Input Pin Connection
Reference
voltage
input
C = 100 to 1000 pF
If there is a possibility that noise equal to or higher than AV
REF
or
equal to or lower than AV
SS
may enter, clamp with a diode with a
small V
F
value (0.3 V or lower).
AV
REF
AV
SS
V
SS
ANI0 to ANI4
(5) ANI0 to ANI4
The analog input pins (ANI0 to ANI4) also function as input port pins (P10 to P14).
When A/D conversion is performed with any of pins ANI0 to ANI4 selected, do not execute a port input instruction
while conversion is in progress, as this may reduce the conversion resolution.
Also, if digital pulses are applied to a pin adjacent to the pin in the process of A/D conversion, the expected A/D
conversion value may not be obtainable due to coupling noise. Therefore, avoid applying pulses to pins adjacent
to the pin undergoing A/D conversion.
(6) AV
REF
pin input impedance
A series resistor string of approximately 21 k
Ω
is connected between the AV
REF
pin and the AV
SS
pin.
Therefore, if the output impedance of the reference voltage is high, this will result in parallel connection to the
series resistor string between the AV
REF
pin and the AV
SS
pin, and there will be a large reference voltage error.
Summary of Contents for mPD780973 Series
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