NEC 78K0 Series User Manual page 323

Reference
voltage
input
C = 100 to 1,000 pF
(5) ANI0/P20 to ANI7/P27
<1> The analog input pins (ANI0 to ANI7) are also used as input port pins (P20 to P27).
When A/D conversion is performed with any of ANI0 to ANI7 selected, do not access P20 to P27 while
conversion is in progress; otherwise the conversion resolution may be degraded. It is recommended to
select pins used as P20 to P27 starting with the ANI0/P20 that is the furthest from AV
<2> If a digital pulse is applied to the pins adjacent to the pins currently used for A/D conversion, the expected
value of the A/D conversion may not be obtained due to coupling noise. Therefore, do not apply a pulse to
the pins adjacent to the pin undergoing A/D conversion.
(6) Input impedance of ANI0 to ANI7 pins
This A/D converter charges a sampling capacitor for sampling during sampling time.
Therefore, only a leakage current flows when sampling is not in progress, and a current that charges the
capacitor flows during sampling. Consequently, the input impedance fluctuates depending on whether sampling
is in progress, and on the other states.
To make sure that sampling is effective, however, it is recommended to keep the output impedance of the analog
input source to within 10 kΩ, and to connect a capacitor of about 100 pF to the ANI0 to ANI7 pins (see Figure 13-
20).
(7) AV pin input impedance
REF
A series resistor string of several tens of kΩ is connected between the AV
Therefore, if the output impedance of the reference voltage source is high, this will result in a series connection to
the series resistor string between the AV
CHAPTER 13 A/D CONVERTER
Figure 13-20. 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 ANI7
AV
SS
V
SS
and AV pins, resulting in a large reference voltage error.
REF SS
Preliminary User's Manual U17260EJ3V1UD
may enter, clamp with a diode with a
SS
value (0.3 V or lower).
and AV
REF SS
or
REF
.
REF
pins.
323