38K2 datasheet(296/358 Pages) RENESAS

296 / 358 page

3.3 Countermeasures against noise

Rev.2.00

Oct 15, 2006

page 35 of 99

38K2 Group

REJ09B0338-0200

APPENDIX

3.3.3 Wiring to analog input pins

• Connect an approximately 100

Ω to 1 kΩ resistor to an analog signal line which is connected to an analog

input pin in series. Besides, connect the resistor to the microcomputer as close as possible.

• Connect an approximately 1000 pF capacitor across the VSS pin and the analog input pin. Besides,

connect the capacitor to the VSS pin as close as possible. Also, connect the capacitor across the analog

input pin and the VSS pin at equal length.

q Reason

Signals which is input in an analog input pin (such as an A/D converter/comparator input pin) are

usually output signals from sensor. The sensor which detects a change of event is installed far

from the printed circuit board with a microcomputer, the wiring to an analog input pin is longer

necessarily. This long wiring functions as an antenna which feeds noise into the microcomputer,

which causes noise to an analog input pin.

If a capacitor between an analog input pin and the VSS pin is grounded at a position far away from

the VSS pin, noise on the GND line may enter a microcomputer through the capacitor.

Fig. 3.3.7 Analog signal line and a resistor and a capacitor

Analog

input pin

VSS

Noise

Thermistor

Microcomputer

N.G.

O.K.

(Note)

Note : The resistor is used for dividing

resistance with a thermistor.

38K2 Datasheet(PDF) 296 Page - Renesas Technology Corp