NEC 78K0/KD1 Series User Manual page 242

(4) Noise countermeasures
To maintain the 10-bit resolution, attention must be paid to noise input to the AV
Because 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 12-21, to reduce noise.
Reference
voltage
input
C = 100 to 1,000 pF
(5) ANI0/P20 to ANI7/P27
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 execute the input instruction to port
2 while conversion is in progress; otherwise the conversion resolution may be degraded.
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
In this A/D converter, the internal sampling capacitor is charged and sampling is performed for approx. one tenth
of the conversion time.
Since only the leakage current flows other than during sampling and the current for charging the capacitor also
flows during sampling, the input impedance fluctuates and has no meaning.
To perform sufficient sampling, however, it is recommended to make the output impedance of the analog input
source 10 kΩ or lower, or attach a capacitor of around 100 pF to the ANI0 to ANI7 pins (see Figure 12-21).
(7) AV pin input impedance
REF
A series resistor string of several tens of 10 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
242
CHAPTER 12 A/D CONVERTER
Figure 12-21. 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 U16315EJ1V0UD
REF
may enter, clamp with a diode with a
SS
value (0.3 V or lower).
and AV
REF
pin and pins ANI0 to ANI7.
or
REF
pins.
SS