AN-1161

Application Note

Rev. 0 | Page 4 of 16

ELECTROMAGNETIC COMPATIBILITY

EMC is the ability of an electronic system to function satis-

factorily in its intended electromagnetic environment without

introducing intolerable electromagnetic disturbances to that

environment. An electromagnetic environment is composed of

both radiated and conducted energy. Therefore, EMC has two

aspects, emission and susceptibility.

Emission is the unwanted generation of electromagnetic energy

by a product. It is often desirable to control emission in order to

create an electromagnetically-compatible environment.

Susceptibility is a measure of the ability of electronic products

to tolerate the influence of electromagnetic energy radiated or

conducted from other electronic products or electromagnetic

influences. Immunity is the opposite of susceptibility.

Equipment that has high susceptibility has low immunity.

The international electrotechnical commission (IEC) is the

world’s leading organization that prepares and publishes

international standards for all electrical, electronic, and related

technologies. Since 1996, all electronic equipment sold to or

within the European community must meet EMC levels as

defined in specification IEC 61000-4-x.

The IEC 61000 specifications define the set of EMC immunity

requirements that apply to electrical and electronic equipment

intended for use in residential, commercial, and light industrial

environments. Within this set of specifications, there are three

types of high voltage transients that electronic designers need to

be concerned about for data communication lines. These are

•

IEC 61000-4-2 Electrostatic Discharge (ESD)

•

IEC 61000-4-4 Electrical Fast Transients (EFT)

•

IEC 61000-4-5 Surge Immunity

This application note deals with increasing the protection level

of RS-485 ports to protect against the these three main EMC

transients.

Each of these specifications defines a test method to assess

the immunity of electronic and electrical equipment against

the defined phenomenon. The following sections provide a

summary of each of these tests.

ELECTROSTATIC DISCHARGE

ESD is the sudden transfer of electrostatic charge between

bodies at different potentials caused by near contact or induced

by an electric field. It has the characteristics of high current in a

short time period.

An object can become charged due to a number of mechanisms.

A charge can occur by simple contact with another charged

object. It can also occur as a result of triboelectric charging,

which is the generation of static electricity caused by rubbing

two substances together. Alternatively, an object can be charged

as a result of induction charging. In this case, there is no

physical contact with the charged object yet charging can occur

if it is within the electric field of the charged object.

The primary purpose of the IEC 61000-4-2 test is to determine

the immunity of systems to external ESD events outside the

system during operation. IEC 61000-4-2 specifies testing using

two coupling methods, contact discharge and air-gap discharge.

Contact discharge implies the discharge gun is placed in direct

connection with the unit under tested. Air gap discharge uses a

higher test voltage, but does not make direct contact with the unit

under test.

During air discharge testing, the charged electrode of the

discharge gun is moved toward the unit under test until a

discharge occurs as an arc across the air gap. The discharge

gun does not make direct contact with the unit under test. A

number of factors affect the results and repeatability of the air

discharge test, including humidity, temperature, barometric

pressure, distance, and rate of approach to the unit under test.

This method is a better representation of an actual ESD event,

but is not as repeatable. Therefore, contact discharge is the

preferred test method.

IEC 61000-4-2 specifies voltage test levels for different environ-

mental conditions along with a current waveform. Table 1

shows the relationship between the environment and the test

voltage. The test levels should be selected in accordance with

the most realistic installation and environment conditions the

final product will be subjected to.

Level 1 is the least severe and Level 4 the most severe. Level 1

and Level 2 are for products installed in controlled environ-

ments that have antistatic material. Level 3 and Level 4 are for

products installed in more severe environments where ESD

events with higher voltages are more common.

Figure 2 shows the 8 kV contact discharge current waveform

as described in the specification. Some of the key waveform

parameters to note are fast rise times of less than 1 ns and short

pulse widths of approximately 60 ns. This equates to a pulse

with total energy in the range of tens of mJ.

The test is performed with single discharges. The test point is

subjected to at least 10 positive and 10 negative discharges. A 1 s

interval between discharges is recommended.