[6] Handling Guide
108
3.3.16
External
Noise
Printed circuit boards with long I/O or signal pattern lines are
vulnerable to induced noise or surges from outside sources.
Consequently, malfunctions or breakdowns can result from
overcurrent or overvoltage, depending on the types of device
used. To protect against noise, lower the impedance of the
pattern line or insert a noise-canceling circuit. Protective
measures must also be taken against surges.
For details of the appropriate protective measures for a
particular device, consult the relevant databook.
3.3.17 Electromagnetic
Interference
Widespread use of electrical and electronic equipment in recent years has brought with it radio and
TV reception problems due to electromagnetic interference. To use the radio spectrum effectively and
to maintain radio communications quality, each country has formulated regulations limiting the
amount of electromagnetic interference which can be generated by individual products.
Electromagnetic interference includes conduction noise propagated through power supply and
telephone lines, and noise from direct electromagnetic waves radiated by equipment. Different
measurement methods and corrective measures are used to assess and counteract each specific type
of noise.
Difficulties in controlling electromagnetic interference derive from the fact that there is no method
available which allows designers to calculate, at the design stage, the strength of the electromagnetic
waves which will emanate from each component in a piece of equipment. For this reason, it is only
after the prototype equipment has been completed that the designer can take measurements using a
dedicated instrument to determine the strength of electromagnetic interference waves. Yet it is
possible during system design to incorporate some measures for the prevention of electromagnetic
interference, which can facilitate taking corrective measures once the design has been completed.
These include installing shields and noise filters, and increasing the thickness of the power supply
wiring patterns on the printed circuit board. One effective method, for example, is to devise several
shielding options during design, and then select the most suitable shielding method based on the
results of measurements taken after the prototype has been completed.
3.3.18 Peripheral
Circuits
In most cases semiconductor devices are used with peripheral circuits and components. The input
and output signal voltages and currents in these circuits must be chosen to match the semiconductor
device’s specifications. The following factors must be taken into account.
(1) Inappropriate voltages or currents applied to a device’s input pins may cause it to operate
erratically. Some devices contain pull-up or pull-down resistors. When designing your system,
remember to take the effect of this on the voltage and current levels into account.
(2) The output pins on a device have a predetermined external circuit drive capability. If this drive
capability is greater than that required, either incorporate a compensating circuit into your
design or carefully select suitable components for use in external circuits.
Input/Output
Signals
