ESD 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.

Should 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.

A 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.

To 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.

Calibrated 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.

The 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.

Measure Current
You’ll 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.

Don’t 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 waveform display.

Two 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.

Second, 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.

Your 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
Before 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.

To 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.

To 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.

When 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.

Current 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.

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 

Figure 3. As much as 30% of the current passing through a current probe can couple into the probe’s cable.

and add it to the current you measure.

Now 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.

After 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
You 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.

Interference 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 ESD.

If 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
Often, 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.

Check 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.

You 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 critical circuits.

So 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.

Ultimately, 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 site.

You 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.

Given 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. T&MW

FOOTNOTE
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.

Doug Smith 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 Symposia. E-Mail:doug@dsmith.org.

How ESD Affects Circuits

An ESD event is a voltage discharge that creates currents in circuits. Those currents can in turn produce voltages across the length of a wire or a circuit trace, and the voltage levels may surprise you. Assume a conductor has an inductance of 10 nH/cm. Therefore, the voltage induced by an ESD current along 1 cm of wire is:

V = L x dI/dt

The current produced by an ESD event can reach 10 A in just 1 ns. That fast rise will induce voltage of 10x10–9 x10/10–9 or 100 V/cm. The figure shows that inductance in a return path between two circuits can cause them to have different potentials relative to ground when the fast current from ESD passes through the ground path. That difference can cause a logic gate to misinterpret a bit.

Significant current also can flow through small capacitances between and across circuit components and signal paths. You can calculate the current using the following equation:

I = C x dV/dt

Assume an ESD pulse causes a change in voltage of 1000 V in 1 ns across 5 pF of capacitance. The pulse will produce 5x10–12x1000/10–9 or 5 A of current in the circuit.—Doug Smith