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3. Wiring for analog input pins, analog power source pins, etc.
(1) Processing analog input pins
Connect a resistor to the analog signal line, which is connected to an analog input pin, and make
the connection as close to the microcomputer as possible.
Connect a capacitor between the analog input pin and AVss pin, as close to the AVss pin as
possible.
Reason: A signal which is input to the analog input pin is usually an output signal from a sensor.
The sensor which measures changes in status tends to be installed far from the microcomputer
printed circuit board. The result is long wiring that becomes an antenna which picks up
noise and feeds it into the microcomputer analog input pin.
If a capacitor between an analog input pin and AVss pin is grounded far away from AVss
pin, noise on the GND line may enter the microcomputer through the capacitor.
Noise
Thermistor
Notes 1 : Design an external circuit for ANi pin so that charge/discharge is available within 1
cycle of
2 : This resistor and thermistor are used to divide resistance.
Fig. 10. Example of noise immunity improvement using thermistor
Appendix 8. Examples of noise immunity improvement
(Note 2)
RI
CI
.
AD
7751 Group User's Manual
M37751
AN
i
Reference value
AVss
RI : Approximately 100 to 1000
CI Approximately 100 to 1000 pF
APPENDIX
:
20–65
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