NEC NEC PD78081(A), User Manual

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CHAPTER 10 A/D CONVERTER
(3) Noise countermeasures
In order to maintain 8-bit resolution, attention must be paid to noise on pins AV
REF
and ANI0 to ANI7. Since
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 10-10 in order to reduce noise.
Figure 10-10. Analog Input Pin Disposition
(4) Pins ANI0/P10 to ANI7/P17
The analog input pins ANI0 to ANI7 also function as input/output port (PORT1) pins. When A/D conversion
is performed with any of pins ANI0 to ANI7 selected, be sure not to execute a PORT1 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.
(5) AV
REF
pin input impedance
A series resistor string of approximately 10 k
Ω
is connected between the AV
REF
pin and the AV
SS
pin.
Therefore, if the output impedance of the reference voltage source 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.
ANI0-ANI7
AV
REF
V
DD
AV
DD
AV
SS
V
SS
Reference
Voltage Input
C=100-1000 pF
If there is possibility that noise whose
level is AV
REF
or higher or AV
SS
or lower may enter,
clamp with a diode with a small V
F
(0.3 V or less).
V
DD