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CHAPTER 14 A/D CONVERTER
(3) Countermeasures against noise
To keep the resolution of 8 bits, noise superimposed on the AV
REF
and ANI0-ANI7 pins must be suppressed
as much as possible. The higher the output impedance of the analog input source, the greater the effect. To
suppress noise, connecting an external capacitor as shown in Figure 14-9 is recommended.
Figure 14-9. Processing Analog Input Pin
(4) ANI0/P10-ANI7/P17
The analog input pins (ANI0-ANI7) are also used as I/O port pins (port 1).
To use these pins as the analog input pins, specify the input mode.
When A/D conversion is performed with any of ANI0-ANI7 selected, do not execute the input instruction for
port 1 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 adjacent pins to the pin under A/D conversion.
(5) Input impedance to AV
REF
pin
A series resistor string of about 10 k
is connected between the AV
REF
and AV
SS
pins.
If the output impedance of the reference voltage source is high, therefore, an error of the reference voltage
increases by connecting the impedance in parallel with the series resistor string between the AV
REF
and AV
SS
pins.
Reference
voltage input
AV
REF
ANI0-ANI7
V
DD
AV
DD
AV
SS
V
SS
C = 100-
1000 pF
If there is a possibility that noise
AV
REF
or higher, or AV or lower is input,
clamp the noise by using a diode with a low V
F
(0.3 V MAX.).
SS
AV
REF