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CHAPTER 12 8-BIT A/D CONVERTER (
µ
PD780344, 780344Y SUBSERIES)
User’s Manual U15798EJ2V0UD
(4) Noise countermeasures
To maintain the 8-bit resolution, attention must be paid to noise input to the AV
DD
and ANI0 to ANI7 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 12-12 to reduce noise.
Figure 12-12. 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
DD
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
DD
AV
SS
V
SS0
or V
SS1
ANI0 to ANI7
V
DD0
or V
DD1
(5) ANI0/P10 to ANI7/P17
The analog input pins (ANI0 to ANI7) also function as input port pins (P10 to P17).
When A/D conversion is performed with any of the ANI0 to ANI7 pins 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 being A/D converted, 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) Input impedance of ANI0 to ANI7 pins
In this A/D converter, the internal sampling capacitor is charged and sampling is performed for approx. one tenth
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 ANI7 pins (see
Figure 12-12
).
(7) AV
DD
pin input impedance
A series resistor string of several 10 k
Ω
is connected between the AV
DD
pin and the AV
SS
pin.
Therefore, when the output impedance of the reference voltage is too high, it seems as if the AV
DD
pin and the
series resistor string are connected in series. This may cause a greater reference voltage error.
Содержание mPD780344 Series
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