7733 datasheet(613/940 Pages) RENESAS

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7733 Group User’s Manual

21-65

APPENDIX

Appendix 8. Countermeasure examples against noise

3. Wiring for analog input pins, analog power source pins, etc.

(1) Processing analog input pins

q Connect a resistor to the analog signal line, which is connected to an analog input pin, in series.

Additionally, connect the resistor to the microcomputer as close as possible.

q Connect a capacitor between pin AVss and the analog input pin, as close to pin AVss as possible.

Reason

A signal which is input to the analog input pin is usually an output signal from a sensor.

The sensor, which detects changes in status, is installed far from the printed circuit board. Therefore,

this long wiring between them becomes an antenna which picks up noise and feeds it into the

microcomputer.

If a capacitor between an analog input pin and pin AVss is grounded far away from pin AVss, noise

on the GND line may enter the microcomputer through the capacitor.

Fig. 17 Countermeasure example against noise for analog input pin using thermistor

ANi

AVss

Thermistor

Noise

M37733MHBXXXFP

Reference values @

RI: Approximate 100

Ω to 1000 Ω

CI: Approximate 100 pF to 1000 pF

Notes 1: Design an external circuit for pin ANi so that charge/discharge is available within

1 cycle of

2: This resistor and the thermistor are used to divide resistance.

(Note 2 j

Not

acceptable

Acceptable

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