Often unanticipated, ESD is a potential source of electromagnetic interference (EMI). As a result, a significant ESD event generates a high-energy EMI pulse that can upset neighboring electronic systems.
The act of grounding an ungrounded ESD-sensitive (ESDS) device can trigger an ESD event, yielding latent or catastrophic damage by means of an energy or voltage failure mechanism in the device. To minimize this potential problem, you must control the rate of discharge during grounding.1
Decreasing the rate of discharge will limit the current density of a potential ESD event. Any combination of an increase in resistance or capacitance in the contacting electrodes (the two materials that sustain a discharge) can decrease the rate of discharge and lessen the effects of an ESD event.
One side effect from ESD is induced EMI. ESD-induced EMI in the near-vicinity of mission-critical equipment can cause data errors, temporary resets, or even power-up resets requiring operator intervention.2 This is prompted when EMI converts to a voltage or current that, in turn, corrupts the operation of the circuit.
The effects from undesired EMI on ungrounded or unshielded conductors are commonly underestimated. An ESD event occurring outside an ESDS protective work area still can pose a risk to unshielded and ungrounded conductors within the ESDS work area.
Case Examples
Some examples of ESD/EMI problems reported by the Center for Devices and Radiological Health are listed by product recall numbers. Recall numbers M485337, M485338, M562311, March 1994, stated that static generated from sheets when a nurse was making a bed caused infusion pumps to sound a processor lock-up alarm.
Recall number M249358, October 1991, stated that a discharge from an operator to the timer of a radiation therapy system caused the timer’s display to blank just as treatment began. Recall numbers Z3112, Z3212, Z3132, Z142, January 1992, reported that ESD affected infant radiant warmers, causing the heater to turn on or off, the alarm not to activate, and the display to become blank or corrupted.3
Today’s TTL and CMOS logic states have a logic 0 at 0.8 V or lower and a logic 1 at 2.0 V or higher. This small indeterminate range of 1.2 V for most TTL and some CMOS logic circuits make them more susceptible to induced EMI voltages.
One example of ESD-induced EMI was characterized by office chairs.4,5 Induced voltages over 2 V have been measured on a printed circuit assembly 90 cm from the furniture ESD.5 Two volts is enough to easily drive a TTL circuit, let alone an ECL circuit, into a logic error.
Table 1 lists some logic devices and their potential susceptibility to EMI. Noise margin is a quantitative measure of a device’s noise immunity. The high-level DC noise margin in Table 1 is the difference between the minimum device output levels for a logic high VOH of the driving gate and the minimum input level VIH required by the driven gate to recognize a logic 1 state. The indeterminate range is the difference between the low-level maximum and high-level minimum of the logic inputs to differentiate between a logic 0 or 1.
Some types of common lab stools and office chairs can radiate a series of impulsive fields from metal legs due to internal ESD when a person rises from the chair. As many as 12 pulses have been recorded within a 10-s period after a person rises from a chair.4
The research reported that a value of tens of millivolts per inch (~2 V/m) generally is not enough to affect digital logic, but values over 1 V/in. (40 V/m) are potential problems. One example observed induced voltages of greater than 4 V/in. (>160 V/m) in cables 1 ft from an office chair.4
What often looks like software errors in process equipment actually may be caused by an external static charge (or discharge) problem. An ESD event anywhere in a room can cause EMI. That EMI can couple into a system through cables or an open chassis to induce a noise voltage greater than the indeterminate range of the logic inputs and cause a single event upset.
Theoretical Energy Analysis
Mechanism of an ESD Event
Three well-known methods can simulate an ESD event: the human body model, the machine model, and the charged-device model. Depending on the application, each has its place in designing the proper ESD control program.
Induced voltage from an EMI energy transfer to a logic input trace with a typical area of 40 mm2 could be as high as 485 mV with an ESD-induced 100 MV/m field at 33 cm (Table 2). As depicted in Table 1, 485 mV is enough voltage to flip the logic state of an ECL device. From the same ESD event, a data input cable with a receiving area of 40 cm2 can have an EMI-induced voltage of 4.85 V, enough voltage to drive a logic error in any family or subfamily of logic circuits.
ESD Event
An ESD event can have a fast rise time, especially for low-voltage discharges.7 The waveform for an ESD event includes components with a frequency range from DC to more than 6 GHz.5 This EMI can readily couple to circuit traces.
For ungrounded conductors coupled within a capacitive circuit, this EMI wave can induce a static charge, building until a discharge, breakdown, recombination, or neutralization occurs. By their nature, high-speed circuits tend to be very susceptible to high-frequency signals such as those from a nearby ESD event.
The electrostatic field strength (Eo) just before an ESD is proportional to the charged voltage (V) at gap width d and defined as Eo =Some Solutions
Assume that all electronic devices are susceptible to damage or logic error states from ESD and EMI, respectively, and take the proper precautions.
Proper grounding of isolated conductors and use of ground planes near active conductors minimize some of these effects.
Shielding of known emitting devices helps, but it is the unknown emitters that cause the most problems. As a result, shielding the sensitive logic devices helps combat EMI-induced logic errors. Start shielding at the device level, for it is less costly than at the system level.
Reduce ground-loop areas between interconnected equipment and systems. Route interconnected cables inside conduit, cable trays, or raceways when possible. Do not coil excess cable into a helix, but rather fold it back and forth.
Metal-to-metal discharges always derive the largest current derivatives (di/dt) and generate the strongest EMI fields. Treat isolated conductors as charged devices and ground them with an electrically dissipative material (R > 104 W ). This slows down the energy transfer from the conducted ESD, causing the resultant EMI to be negligible to any active near- or far-field system.
Conclusion
High-energy ESD can drive EMI energy to couple and charge passive circuits or energize active circuits with significant system problems. It is not only necessary to account for EMI from known sources, but you also must consider unplanned sources such as ESD events in the vicinity of the active or sensitive systems. With today’s logic devices having smaller noise margins and indeterminate ranges, susceptibility to ESD-induced EMI should be accounted for in the design and implementation of systems incorporating logic circuits.
References
NOTE: This article can be accessed on EE’s TestSite at www.nelsonpub.com/ee/. Select EE Archives and use the key word search.
2. Chase, G., “EMI from ESD—An Insidious Alliance,” NARTE News, Vol. 14, No. 1, 1996, p. 22.
3. Silberberg, J., “What Can/Should We Learn from Reports of Medical Device Electromagnetic Interference?,” FDA, Rockville, EMBC95 Paper 10.2.1.3, 1995.
4. Smith, D., “A New Type of Furniture ESD and Its Implications,” EOS/ESD Symposium Proceedings, EOS-15, 1993.
6. Dorf, R.C., The Electrical Engineering Handbook, 2nd Edition, CRC Press, pp. 1773-1777, 1997.
7. Podgroski, S., Dunn, J., and Yeo, R., “Study of Picosecond Rise Time in Human-Generated ESD,” Proceedings of the IEEE International Symposium on Electromagnetic Compatibility, Cherry Hill, NJ, Aug. 12-16, 1991, pp. 263-264.
8. Pommerenke, D., “Transient Fields of ESD,” ESO/ESD Symposium Proceedings, 1994, pp. 150-159.
About the Author
Ryne C. Allen is the technical manager at ESD Systems, a division of Desco Industries. Previously, he was chief engineer and lab manager at the Plasma Science and Microelectronics Research Lab at Northeastern University. Mr. Allen is a NARTE-certified ESD control engineer and the author of 23 published papers. He graduated from Northeastern University with B.S.E.E., M.S.E.E., and M.B.A. degrees. ESD Systems, 19 Brigham St., Unit 9, Marlboro, MA 01752, (508) 485-7390, www.esdsystems.com.
Table I
|
Logic Family
|
Static Power Dissipation Per Gate (mW) |
High-Level DC Noise Margin (mV) |
Input Logic Indeterminate Range (V) |
|
ECL-10 |
25 |
145 |
0.37 |
|
TTL |
10 |
400 |
1.2 |
|
ALS-TTL |
1.2 |
500 |
1.2 |
|
AS-TTL |
2 |
500 |
1.2 |
|
LS-TTL |
2 |
500 |
1.2 |
|
HC-CMOS |
0.003 |
175 |
2.25 |
|
HCT-CMOS |
0.003 |
165 |
1.2 |
Table 2
|
Field Strength From ESD Event |
Induced Voltage on Receiving Area |
||||
|
E (V/m) |
40 mm2 |
4 cm2 |
40 cm2 |
400 cm2 |
0.4 m2 |
|
100 MV/m |
485 mV |
1.53 V |
4.85 V |
15.3 V |
48.5 V |
|
1 MV/m |
4.85 mV |
15.3 mV |
48.5 mV |
153 mV |
485 mV |
|
1 kV/m |
4.85 m V |
15.3 m V |
48.5 m V |
153 m V |
485 m V |
Copyright 1998 Nelson Publishing Inc.
May 1998