is a powerful source of interference that can cause electronic systems
to reset, lose data, or fail to operate. While standards require that
you prove your product can withstand ESD, you should also test your
product under conditions that more closely simulate the environment
in which your customer will use it.
your product continue to operate flawlessly at both your site and
at your customer’s site after ESD tests, then you have a robust product.
If your system operates at subpar levels during or after an ESD test,
however, then you need to investigate how the energy from an ESD event
enters your system. You can perform your investigations by measuring
the current that ESD exerts on your system.
typical ESD event can send several amperes of current driven by thousands
of volts into your product. Those voltages and currents can cause
a product to function improperly or they can fatally damage circuits.
(" How ESD Affects Circuits ,"
below, explains these voltages and currents in more detail.) The damage
that ESD inflicts on circuits can also induce problems in your test
equipment. And test equipment can place an additional load on your
EUT, which can alter your EUT’s ESD susceptibility.
measure the effects of ESD in electronic equipment, you need
several tools, including at least one ESD simulator, an oscilloscope,
and current probes. Plus, you need BNC cables to connect
the probes to your scope. Be sure to use high-quality, double-shielded
cables to help keep ESD-generated EMI out of the measurements.
ESD simulators on the market today meet the requirements
of IEC 61000-4-2, and they cost from $5000 to $10,000, depending
on options and accessories. Unfortunately, the IEC 61000-4-2
specification only roughly specifies the ESD waveform that
the simulators must generate.1 The standard specifies the
initial risetime, first peak current, and the currents at
30 ns and 60 ns after the initial pulse reaches 10% of its
full amplitude. But the standard doesn’t specify the ESD
current at other points. Therefore, two simulators can meet
IEC 61000-4-2 and yet have significantly different waveforms.
For instance, one simulator on the market has a smooth waveform
after the initial peak, whereas another compliant simulator
has a fair amount of high-frequency (above 1 GHz) ringing.
variations in simulator current waveforms can result in a
product passing CE Marking ESD requirements with one simulator,
yet failing with another. There’s no easy answer to this
problem. You can test with multiple simulators, but doing
so will at least double both the required test time and the
cost of your ESD simulator investment. Or you can perform
all of your test and debug procedures with the same simulator.
At least your results will be consistent, although they may
not agree with tests performed using another simulator.
need to measure the current produced by an ESD event. That calls for a
fast oscilloscope. Your scope should have a minimum digitizing
rate of 2 Gsamples/s and have a vertical amplifier bandwidth
of at least 500 MHz. At that level of performance, you can
generally troubleshoot products incorporating logic families
that have risetimes of 1 ns or longer. If you can, use an
8-Gsample/s scope with a bandwidth greater than 1 GHz. The
higher sampling rate lets you see the ESD pulse in greater
detail, and it lets you more accurately measure a pulse’s
risetime. Prices have fallen, and you can get such a scope
for what a 2-Gsample/s scope cost just a few years ago.
ever assume your test equipment is immune to ESD. Oscilloscopes
contain sensitive analog circuits, and just a small leakage
of ESD energy into these circuits can result in an inaccurate
signs will alert you that ESD is affecting your scope. First,
ESD can cause the scope to trigger even though the waveform
on the display doesn’t pass through the trigger level. If
you’re using pulse-width triggers, the scope may trigger
even if the displayed waveform doesn’t meet the width parameter
you’ve set. ESD may have entered the scope’s trigger circuit
and initiated an unwanted trigger.
if you change the scope’s vertical scale and the amplitude
of the displayed waveform does not change by the factor you
expect, you have an ESD problem. How so? If ESD-generated
EMI enters the scope’s vertical amplifiers at a point after
the scope’s variable attenuator, the displayed signal may
not change at all when you change the vertical sensitivity.
Be sure that when you change the scope’s vertical sensitivity,
you see the displayed signal change by the same factor. If
it does and the triggering seems to be working properly,
you’re ready to start measuring—after you take some precautions.
If the signal doesn’t change by the same factor, you can
add six or more lossy-core ferrites to each scope-probe cable,
or you can extend the cables to move the scope farther from
your ESD source.
probes and their cables can pick up ESD-induced EMI. To combat
that possibility, pass each probe cable through one or two
ferrite cores at the scope end. In some cases, you might
have to connect the probe cable’s shield to the chassis of
the EUT, which might direct the current from the discharge
directly into the probe cable. In that situation, use five
or six ferrite cores on the probe’s cable.
Null Things Out
you set out to measure the effects of ESD, you should check your
measurements for accuracy. I call these checks "null
experiments" because their result is usually zero. Any
deviation from zero sets the error level of the measurement,
and you must take that error into account.
measure currents originating from ESD, I use current probes
with my scope because they don’t directly connect to the
circuit under test and they can’t place an additional load
on a circuit. Pay attention to a current probe’s transfer
impedance—the ratio of voltage out of the probe to current
flowing through the probe. Typical probes you might use for
ESD investigations have transfer impedances that range from
about 1 W to about 5 W.
|Figure 1 Bend a current-carrying wire through a current probe to measure the probe's electric-field response. |
Most current probes were designed for measuring radio-frequency currents in low-impedance (50 W to 100 W) systems, which produces a significant current for a relatively low voltage. In a 50-W system, you get 1 A of current per 50 W. That much current causes magnetic
fields to dominate the electric fields around the cable.
The conductor’s voltage can jump a few thousand volts with
only a few amperes flowing through it. Thus, electric
fields can dominate magnetic fields. The electric fields
can couple through the probes and produce measurement errors.
Therefore, your current probes need very good electric field
shielding. Figure 1 shows how to check the electric field response of a current probe using a folded-wire null experiment.
perform the experiment, fold the wire carrying the ESD current
and insert it into the current probe until the fold is just
flush with the opposite side of the probe. Then inject ESD
through the wire. You’ll expose the inside of the probe to
the electric field on the wire but no net current flows through
the probe. The probe’s output should read zero. I’ve seen
some current probes in this configuration that produce an
output half as great as the output produced when the current-carrying
wire passes through the probe. Such a current probe isn’t
particularly useful in ESD investigations.
you measure ESD currents, you’ll often measure the current
both inside and outside your EUT’s enclosure. That dual measurement
requires two current probes. Most current probes have an
amplitude accuracy of ±2 dB, which is acceptable for
many EMI emissions tests in which you measure the result
on a spectrum analyzer at 10 dB/div. The ±2 dB specification
won’t let you accurately compare the amplitude of ESD currents
from the two probes.
|Figure 2. Use matched current probes to measure the current in a conductor and the current in a current probe’s cable.|
For that measurement, you’ll need a pair of matched
current probes. Some current probe manufacturers will provide sets
of current probes with the amplitude response matched to
within 1% or 2%. A manufacturer may not charge more for the
matched probes, but you must remember to ask for them.
probes can cause measurement errors, so you need to know
how much error your probes contribute. Figure 2
shows two current probes set up to make an error measurement.
An ESD-induced current flows down the cable that passes through
the current probe on the right. A second, matched current
probe on the left measures the current flowing from the
righthand probe. The ESD current can couple into the righthand
probe through the capacitance between the probe on the right
and the cable passing through it.
shows the output of the two current probes. The top waveform
shows the ESD current flowing in the vertical wire as measured
by the righthand probe of Figure 2. The bottom waveform shows
the current measured by the lefthand current probe, which
is the common-mode current through the cable passing through
the righthand probe. About 32% of the 8.5 V (or 2.75 V)—divided
by the probe’s transfer impedance—of the high-frequency current
couples onto the probe body due to capacitance. The righthand
probe has removed current from the signal you want to measure.
Only a few picofarads of capacitance can cause these
effects, so you must realize that not all of the original ESD-induced
current may enter the EUT. Thus, if your EUT performs better when
subjected to ESD while your test equipment is connected to it, then you
must measure the diverted current
and add it to the
current you measure.
|Figure 3. As much as 30% of the
current passing through a current probe can couple into the probe’s
that you have the test equipment and you have measured any
errors due to coupling, you can begin your investigation.
I suggest you use IEC 61000-4-2 as a guide for any ESD investigation,
because your product must comply with this standard in order
to bear the CE Marking, which is essential for selling electronics
products in Europe. (A complete discussion of IEC 61000-4-2
is beyond the scope of this article.) The standard dictates
the number of points on your EUT that you must subject to
ESD. By changing the selection of test points, you can affect
the outcome of the test, because some test points may be
more susceptible to ESD interference than others.
you select the test points, you’re ready to subject your
system to ESD. Fire the ESD simulator while you touch its
output electrode directly to the EUT. That’s known as a contact
discharge. IEC 61000-4-2 specifies that you must make 10
discharges to each point for each of four ESD voltages you
use and for both polarities (+ and –). So the number of test
points can significantly affect how long the test takes.
For 4 voltages and 20 test points, that totals 1600 discharges
(4 voltages x 20 points x 2 polarities x 10 discharges).
Such a test could take an hour or more depending on how quickly
you can position the simulator for the next discharge. And
20 points is a small number; some EUTs can require many more
discharge points. For more thorough testing, you can always
perform more ESD testing than the standard requires, but
this will increase your test time.
Here Comes the Judgment
need to use judgment when you select test points. The test points
should represent a reasonably complete test yet minimize test time.
In general, for commercial information-technology equipment, test
at any point that people can touch with their hands. Include places
such as front panels, buttons, conducting enclosures, and cable connector
shells in your set of test points.
from ESD can enter your EUT not only from direct discharges
at the EUT’s enclosure, but also through its cables. To conduct
a valid ESD test, terminate an EUT’s cables in ways similar
to those found in normal operation. Cable terminations affect
both the differential impedance and the common-mode impedance
of the cables. If your EUT connects to other equipment during
actual use, then you must simulate the auxiliary equipment.
You must verify that the currents flowing in the EUT’s cables
are the same as those that flow in normal use. Otherwise,
your test setup might alter the system’s ESD performance,
and it won’t represent how the EUT will respond to real-world
the impedances of the cable terminations in your test setup
differ from those in an actual setup, they’ll most likely
change the normal ringing frequency of signals. The terminations
can dampen cable currents or change the characteristics of
the ESD-induced current’s rising edge. Most cables will attenuate
the ESD pulse’s rising-edge energy before the current reaches
the end of the cable. And the cable itself will attenuate
the high-frequency components of the ESD current. So, if
you don’t properly simulate the cable connections to your
EUT’s peripheral equipment, you won’t get the same attenuation.
Less attenuation will cause more ESD energy to enter the
EUT, which will cause your EUT to falsely fail an ESD test.
More attenuation could result in a false failure, which means
your EUT could be more susceptible to ESD than you realize.
Shield the Cables
you rely on shielded cables to minimize the effects of ESD and other
forms of EMI in a product. A properly terminated cable shield will
minimize the ESD induced current that enters the EUT from a cable.
You can check how well a shielded cable performs by using a pair of
matched current probes.
the amount of current on the shield compared to the amount
of current that enters the EUT through the cable’s connection
at the EUT. Put one current probe on the cable outside of
the EUT. Place the second current probe on the cable or leads
from the cable inside the EUT. Inject an ESD pulse into the
cable’s connector shield at the outside of the EUT. Use the
two probes to see how much of the ESD-induced current in
the cable passes into the EUT. If a measurable current enters
the EUT, then the shield may not cover 360° around the cable’s
wires. Remember to calculate the L3dI/dt voltage drop across
10 nH (about 1 cm) of conductor inside the EUT, which is
a good measure of the ESD interference. If the voltage drop
is large enough to degrade the EUT’s performance, then you’ve
most likely found your ESD problem.
must take care to ensure the internal current probe does
not bring in ESD via its own cable shield. You should ground
the cable’s shield where it enters the EUT’s metal enclosure.
If the system doesn’t have a metal enclosure, you may not
be able to keep EMI currents on the probe cables from reaching
far, I’ve discussed measurements related to a direct hit
from ESD, but ESD can produce EMI that can radiate into an
enclosure through access openings. Once inside an enclosure,
the EMI radiation can couple into wires, PCB traces, and
conductive cabinets, which produces unwanted current pulses.
These currents can induce substantial dI/dt induced voltages
of several volts per centimeter on cables. You can test for
ESD coupling by discharging an ESD simulator into a metal
plate placed close to the EUT. The plate will radiate EMI
from the pulse into your system.
your EUT’s performance determines if it passes an ESD test.
You can easily spot a malfunction in some equipment. In other
equipment, usually complex equipment such as a server,
problems that aren’t immediately obvious may lurk after an
ESD event. Unfortunately, you may not detect these problems
during the ESD test. Instead, they may occur at the customer’s
may not be able to operate the EUT under the same conditions
as your customer. The EUT may have millions of possible operating
states, and some may fail because of ESD damage while other
states function properly. You may not be able to have the
system in a sensitive state during the ESD test, especially
since you probably won’t know beforehand which states the
ESD damage affects. Equipment characteristics combined with
the high-frequency effects of ESD have given rise to the
belief that ESD testing isn’t repeatable. ESD testing is
a matter of cost vs. test coverage. You have to decide how
much testing is cost effective.
the nonrepeatable nature of ESD, you should test your product
under conditions that go beyond the requirements of ESD test
standards. So if you can, perform more discharges at more
voltages than the standards require. Also, don’t rely on
normal operation of the equipment as evidence of the absence
of problems. If your EUT has a microprocessor or operates
under PC control, then you can write diagnostic software
to look for possible problems that can affect your customer.
1. IEC 61000-4-2 (1999-05), Electromagnetic
compatibility (EMC) - Part 4-2: Testing and measurement techniques
- Electrostatic discharge immunity test, International Electrotechnical
Commission, Geneva, Switzerland, www.iec.ch.
is manager of EMC at Auspex Systems, Santa Clara, CA, as well as an
independent consultant. He has published dozens of technical
articles and conference papers at IEEE EMC Symposia and ESD