7547 datasheet(65/91 Pages) RENESAS

65 / 91 page

Rev.1.21

Nov 15, 2006

page 65 of 89

REJ03B0156-0121

7547 Group

Fig. 86 Bypass capacitor across the VSS line and the VCC line

2. Connection of bypass capacitor across VSS line and VCC line

Connect an approximately 0.1

µF bypass capacitor across the VSS

line and the VCC line as follows:

• Connect a bypass capacitor across the VSS pin and the VCC pin

at equal length.

• Connect a bypass capacitor across the VSS pin and the VCC pin

with the shortest possible wiring.

• Use lines with a larger diameter than other signal lines for VSS

line and VCC line.

• Connect the power source wiring via a bypass capacitor to the

VSS pin and the VCC pin.

VSS

VCC

VSS

VCC

N.G.

O.K.

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.

Signals which is input in an analog input pin (such as an A/D con-

verter/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 func-

tions as an antenna which feeds noise into the microcomputer,

which causes noise to an analog input pin.

Fig. 87 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.

• The analog input pin is connected to the capacitor of a voltage

comparator. Accordingly, sufficient accuracy may not be ob-

tained by the charge/discharge current at the time of A/D

conversion when the analog signal source of high-impedance is

connected to an analog input pin. In order to obtain the A/D con-

version result stabilized more, please lower the impedance of an

analog signal source, or add the smoothing capacitor to an ana-

log input pin.

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