CHAPTER 10 A/D CONVERTER
Preliminary User’s Manual U16846EJ1V0UD
198
(4) Noise
countermeasures
To maintain the 10-bit resolution, attention must be paid to noise input to the AV
REF
and ANI0 to ANI3 pins.
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 10-19, to reduce noise.
Figure 10-19. Analog Input Pin Connection
Reference
voltage
input
C = 100 to 1,000 pF
If there is a possibility that noise equal to or higher than AV
REF
or
equal to or lower than AV
SS
may enter, clamp with a diode with a
small V
F
value (0.3 V or lower).
AV
REF
AV
SS
V
SS
ANI0 to ANI3
(5)
ANI0/P20 to ANI3/P23
<1> The analog input pins (ANI0 to ANI3) are also used as input port pins (P20 to P23).
When A/D conversion is performed with any of ANI0 to ANI3 selected, do not access port 2 while
conversion is in progress; otherwise the conversion resolution may be degraded.
<2> If a digital pulse is applied to the pins adjacent to the pins currently being 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 ANI3 pins
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 k
Ω
or lower, or attach a capacitor of around 100 pF to the ANI0 to ANI3 pins (see Figure 10-19).
(7) AV
REF
pin input impedance
A series resistor string of several tens of 10 k
Ω
is connected between the AV
REF
and AV
SS
pins.
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
REF
and AV
SS
pins, resulting in a large reference voltage error.
