Standards efforts target ESD simulator repeatability
Staff -- Test & Measurement World, 1/2/2002 4:15:00 PM
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From the ESD advertising supplement to Test & Measurement World, August 2001.  Standards efforts target ESD simulator repeatability |
» Over the past few years, ESD immunity testing has become a key part of the certification of virtually all electronic products. The European Union’s EMC Directive now requires ESD immunity testing as a precondition to obtaining the CE Mark. Without that mark, you cannot ship electronic products to any member of the Union. To meet the immunity requirement, you must perform ESD tests in accord with standards that include IEC-801-2, IEC 61000-4-4, or EN 61000-4-2. These testing requirements are showing up in markets outside Europe, so testing is a must. Outdated approach Basic EMC standards, in particular IEC 61000-4-2, define the methods you must use to generate reproducible electrical stresses for test purposes. When these standards were developed, the bandwidths of test instruments were smaller than that are now. Thus, the older instruments could not accurately measure the characteristics of—let alone detect—high-bandwidth components in test signals. Recently, a working group within the ESD Association (Rome, NY, www.esda.org) determined that some ESD simulators will produce waveforms that meet the requirements outlined in IEC 61000-4-2, but the waveforms include ringing on the signal and high-frequency components. In effect, different ESD simulators could produce different types of signals and still meet the minimum requirements outlined in IEC 61000-4-2. Typically, you verify an ESD simulator’s performance by using a defined target, essentially a current-monitoring element embedded in a sheet of material that forms a conducting plane. You discharge an ESD simulator on the target and examine the resulting waveform on an oscilloscope. Then you compare the recorded waveform to a reference standard. But the targets built to the original IEC 61000-4-2 standard for the test often exhibited poor performance. “They didn’t produce a flat response out to 1 GHz, and beyond 1 GHz they had terrible frequency response,” explains Hugh Hyatt, president of Hyger Physics (Bremerton, WA) and a member of the ESD Association’s Working Group 14.0. “So for any pulse that contained a high-frequency component, or a spectrum of high-frequency components, you didn’t get an output that made much sense.” Until about five years ago, scopes with a 1-GHz bandwidth were expensive or unavailable. So, many test engineers used 500-MHz scopes. “That’s the same as putting a filter on the front end of the measurement,” notes Hyatt. The ESD Association’s Working Group 14.0 is currently working on a new document, “System Level ESD Simulator Verification” (DS14.0-1999), that will increase the bandwidth spec. The document—still in draft form—will specify a bandwidth that will produce flat—or close to flat—frequency-response measurement up to 1 GHz. The measurements require sophisticated and expensive test equipment, so we’re trying to determine the best bandwidth for the standard that will still let people make meaningful measurements, says Jon Barth, president of Barth Electronics (Boulder City, NV). Barth also serves as a member of the subcommittee working on the draft. Information from the draft is available in a new version of ANSI C63.16, which is still working its way through the approval process. And the ESD Association draft will likely have some impact on a revision of IEC 61000-4-2, which is currently in progress, but it may take two or three years for IEC 61000-4-2 to appear in final form. The DS14.0-1999 draft outlines a time-domain measurement technique that designers, manufacturers, and calibration facilities can use to verify compliance with discharge-current specs defined in system-level ESD standards. The draft defines a measurement system that includes an oscilloscope or digitizer, an ESD target, a target adapter, attenuators, a pulse generator, and appropriate connectors or cables.
In general, the scope limits measurement uncertainty, so the draft calls for the use of a single-shot scope with a bandwidth of at least 1 GHz and a rise time of less than one third the rise time of the simulator pulse you need to measure. If the pulse is repeatable, then you must use a sampling scope with a minimum bandwidth of 1 GHz and a minimum sampling rate of 3 Gsamples/s. DS 14.0-1999 outlines the test setup and procedures required to obtain a reference waveform and a system response waveform (Figure 1). You select a reference voltage for the pulse generator and store the pulse waveform on the scope. Then you insert an ESD target in the circuit and perform the same measurement. After you acquire these waveforms, you must compare the current waveforms and their derivatives with specified performance requirements (Figure 2). “dI/dt is certainly a useful parameter, but in practice it’s likely to be difficult to calculate unless the working group can come up with an inexpensive software package we can recommend,” notes Barth. In the meantime, there is already widespread recognition that over the long term the minimal 1-GHz response defined in the current draft will eventually have to extend to higher frequencies. “There have been several targets built that offer flat response out to 5 GHz, and some are flat out to 10 GHz,” notes Hyatt. “When you put these targets in the test circuit and use a high bandwidth scope, you find that none of the ESD simulators produce the same pulse.” |
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Copyright 2001, Test & Measurement World. Published by Cahners Business Information, Newton, MA. |
For more information
“System Level Electrostatic Discharge (ESD) Simulator Verification Standard, Part 1, Discharge Current,” DS14.0-1999, ESD Association.
Barth, Jon, Dave Dale, Ken Hall, Hugh Hyatt, Darren McCarthy, Joe Nuebel, and Doug Smith, “Measurements of ESD HBM Events, Simulator Radiation and Other Characteristics Toward Creating a More Repeatable Simulator or Simulators Should Simulate” EOS/ESD Symposium Proceedings, 1996, p. 211.
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