Analog Input Pin Disposition - NEC PD78081(A) User Manual

(3) Noise countermeasures
In order to maintain 8-bit resolution, attention must be paid to noise on pins AV
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.
Reference
Voltage Input
C=100-1000 pF
(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 pin input impedance
REF
A series resistor string of approximately 10 k
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
error.
132
CHAPTER 10 A/D CONVERTER
Figure 10-10. Analog Input Pin Disposition
If there is possibility that noise whose
level is AV
clamp with a diode with a small V
V
DD
is connected between the AV
pin and the AV
REF
or higher or AV or lower may enter,
REF SS
AV
REF
ANI0-ANI7
V
DD
AV
DD
AV
SS
V
SS
REF
pin, and there will be a large reference voltage
SS
and ANI0 to ANI7. Since
REF
(0.3 V or less).
F
pin and the AV pin.
SS