NEC PD78F9488 - Manual (Page 184)

CHAPTER 10 10-BIT A/D CONVERTER
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User’s Manual U15331EJ4V1UD
(6) Noise prevention
To maintain a resolution of 10 bits, watch for noise at the AV
DD
and ANI0 to ANI7 pins. The higher the output
impedance of the analog input source, the larger the effect by noise. To reduce noise, attach an external
capacitor to the relevant pins as shown in Figure 10-10.
Figure 10-10. Analog Input Pin Handling
C = 100 to 1,000 pF
If noise greater than or equal to AV DD or less than or
equal to AV SS is likely to come to the ANI0 to ANI7 pins,
clamp the voltage at the pin by attaching a diode with
a small V
F
(0.3 V or lower).
AV SS
V
SS
AV
DD
ANI0 to ANI7
V DD
(7) ANI0 to ANI7
The analog input pins (ANI0 to ANI7) are alternate-function pins. They are also used as port pins (P60 to
P67).
If any of ANI0 to ANI7 has been selected for A/D conversion, do not execute input instructions for the ports;
otherwise the conversion resolution may be reduced.
If a digital pulse is applied to a pin adjacent to the analog input pins during A/D conversion, coupling noise
may occur that prevents an A/D conversion result from being obtained as expected. Avoid applying a digital
pulse to pins adjacent to the analog input pins during A/D conversion.
(8) Input impedance of ANI0 to ANI7 pins
This A/D converter charges the internal sampling capacitor for about 1/10 of the conversion time, and
performs sampling.
Therefore at times other than sampling, only the leak current flows. During sampling, the current for charging
the capacitor also flows, so the input impedance fluctuates and has no meaning.
However, to ensure adequate sampling, it is recommend that the output impedance of the analog input
source be set to 10 k
Ω or lower, or a capacitor of about 100 pF to the ANI0 to ANI7 pins (see to Figure 10-
10).