NEC mPD780024AS Series Preliminary User's Manual page 187

CHAPTER 11 8-BIT A/D CONVERTER ( µ PD780024AS SUBSERIES)
(4) Noise countermeasures
To maintain the 8-bit resolution, attention must be paid to noise input to pin 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 11-16 to reduce noise.
Reference
voltage
input
C = 100 to 1000 pF
(5) ANI0 to ANI3
The analog input pins (ANI0 to ANI3) also function as input port pins (P10 to P13).
When A/D conversion is performed with any of pins ANI0 to ANI3 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 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 pin input impedance
REF
A series resistor string of several 10 kΩ is connected between the AV
Therefore, when the output impedance of the reference voltage is too high, it seems as if the AV
series resistor string are connected in series. This may cause a greater reference voltage error.
Figure 11-16. Analog Input Pin Connection
If there is a possibility that noise equal to or higher than AV
equal to or lower than AV
small V
F
AV
REF
ANI0 to ANI3
V
DD0
AV
DD
AV
SS
V
SS0
Preliminary User's Manual U16035EJ1V0UM
and pins ANI0 to ANI3. Because
REF
may enter, clamp with a diode with a
SS
value (0.3 V or lower).
pin and the AV
REF
or
REF
pin.
SS
pin and the
REF
187