CHAPTER 10 A/D CONVERTER
User’s Manual U16898EJ3V0UD
173
(4) Noise countermeasures
To maintain the 10-bit resolution, attention must be paid to noise input to the AV
REF
pin and ANI0 to ANI3 pins.
<1> Connect a capacitor with a low equivalent resistance and a high frequency response to the power supply.
<2> 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 9-19, to reduce noise.
<3> Do not switch the A/D conversion function of the ANI0 to ANI3 pins to their alternate functions during
conversion.
<4> The conversion accuracy can be improved by setting HALT mode immediately after the conversion starts.
Figure 10-19. Analog Input Pin Connection
Reference
voltage
input
C = 0.01 to 0.1 F
If there is a possibility that noise equal to or higher than AV
REF
or
equal to or lower than V
SS
may enter, clamp with a diode with a
small V
F
value (0.3 V or lower).
AV
REF
V
SS
ANI0 to ANI3
µ
(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 P20 to P23 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 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 during sampling 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 both during sampling and otherwise.
If the shortest conversion time of the reference voltage is used, to perform sufficient sampling, it is recommended
to make the output impedance of the analog input source 1 k
Ω
or lower, or attach a capacitor of around 0.01
µ
F
to 0.1
µ
F to the ANI0 to ANI3 pins (see
Figure 10-19
).