207
CHAPTER 12 10-BIT A/D CONVERTER (
µ
PD780034AS SUBSERIES)
Preliminary User’s Manual U16035EJ1V0UM
(4) Noise countermeasures
To maintain the 10-bit resolution, attention must be paid to noise input to pin AV
REF
and pins ANI0 to ANI3.
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 12-16 to reduce noise.
Figure 12-16. Analog Input Pin Connection
Reference
voltage
input
C = 100 to 1000 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
V
DD0
AV
DD
AV
SS
V
SS0
ANI0 to ANI3
(5) ANI0 to ANI3
The analog input pins (ANI0 to ANI3) also function as port pins.
When A/D conversion is performed with any of pins ANI0 to ANI3 selected, do not execute an input instruction
to port 1 while conversion is in progress, 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.
(6) AV
REF
pin input impedance
A series resistor string of several 10 k
Ω
is connected between the AV
REF
pin and the AV
SS
pin.
Therefore, when the output impedance of the reference voltage is too high, it seems as if the AV
REF
pin and the
series resistor string are connected in series. This may cause a greater reference voltage error.
(7) Interrupt request flag (ADIF0)
The interrupt request flag (ADIF0) is not cleared even if the analog input channel specification register 0 (ADS0)
is changed.
Therefore, if an analog input pin is changed during A/D conversion, the A/D conversion result and conversion
end interrupt request flag for the pre-change analog input may be set just before the ADS0 rewrite. Caution is
therefore required since, at this time, when ADIF0 is read immediately just after the ADS0 rewrite, ADIF0 is set
despite the fact that the A/D conversion for the post-change analog input has not ended.
When A/D conversion is restarted after it is stopped, clear ADIF0 before restart.
