CHAPTER 13 A/D CONVERTER
187
(2) Input range of ANI0 to ANI7
The input voltages of ANI0 to ANI7 should be within the specification range. In particular, if a voltage above
AV
REF
or below AV
SS
is input (even if within the absolute maximum rating range), the conversion value for
that channel will be indeterminate. The conversion values of the other channels may also be affected.
(3) Noise countermeasures
In order to maintain 8-bit resolution, attention must be paid to noise on pins AV
REF
and ANI0 to ANI7. Since
the effect increases in proportion to the output impedance of the analog input source, it is recommended
that a capacitor is connected externally as shown in Figure 13-9 in order to reduce noise.
Figure 13-9. Analog Input Pin Disposition
Reference
voltage input
AV
REF
ANI0 to ANI7
V
DD
AV
DD
AV
SS
V
SS
C = 100 to
1000 pF
If there is possibility to enter noise whose
level is AV
REF
or higher or AV
SS
or lower,
clamp with a diode with a small V
F
(0.3 V or less).
AV
REF
(4) Pins ANI0/P10 to ANI7/P17
The analog input pins ANI0 to ANI7 also function as input/output port (PORT1) pins. Pins used as the analog
input should be specified to the input mode. When A/D conversion is performed with any of pins To execute
A/D conversion by selecting any of ANI0 to ANI7, do not execute an instruction that inputs data to PORT
1 during conversion, as this may reduce the conversion resolution.
Also, if digital pulses are applied to a pin adjacent to the pin in the process of A/D conversion, the expected
A/D conversion value may not be obtainable due to coupling noise. Therefore, avoid applying pulses to
pins adjacent to the pin undergoing A/D conversion.
(5) AV
REF
pin input impedance
A series resistor string of approximately 10 k
is connected between the AV
REF
pin and the AV
SS
pin.
Therefore, if the output impedance of the reference voltage source is high, this will result in parallel
connection to the series resistor string between the AV
REF
pin and the AV
SS
pin, and there will be a large
reference voltage error.