UPD78F0138 NEC, UPD78F0138 Datasheet - Page 268

• Manufacturer Part Number: UPD78F0138
• Description: (UPD78xxxx) 8-Bit Single-Chip Microcontrollers
• Manufacturer: NEC

(4) Noise countermeasures
(5) ANI0/P20 to ANI7/P27
(6) Input impedance of ANI0 to ANI7 pins
(7) AV
268
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 12-19, to reduce noise.
Reference
<1> The analog input pins (ANI0 to ANI7) are also used as input port pins (P20 to P27).
<2> If a digital pulse is applied to the pins adjacent to the pins currently used for A/D conversion, the expected
In this A/D converter, the internal sampling capacitor is charged and sampling is performed for approx. one sixth
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 12-19).
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
C = 100 to 1,000 pF
voltage
REF
input
When A/D conversion is performed with any of ANI0 to ANI7 selected, do not access port 2 while
conversion is in progress; otherwise the conversion resolution may be degraded.
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.
pin input impedance
Figure 12-19. Analog Input Pin Connection
CHAPTER 12 A/D CONVERTER
REF
User’s Manual U16228EJ2V0UD
and AV
AV
AV
V
ANI0 to ANI7
SS
REF
SS
If there is a possibility that noise equal to or higher than AV
equal to or lower than AV
small V
SS
pins, resulting in a large reference voltage error.
F
value (0.3 V or lower).
SS
may enter, clamp with a diode with a
REF
and AV
REF
pin and pins ANI0 to ANI7.
SS
pins.
REF
or