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EMC Basics #5: I/O as critical circuits

[Editor's note : we are pleased to continue our series on the vital and sometimes unappreciated topic of electromagnetic compatibility (EMC), presented by well-known expert Daryl Gerke of Kimmel Gerke Associates. There is a link to previous entries at the end of this item.]

Last, but not least, we like to look at the I/O (Input/Output) circuits during EMI circuit board reviews. As a port of entry for external currents, I/O circuits are particularly vulnerable to threats like ESD (electrostatic discharge) and RFI (radio frequency interference). Radiated emissions are also a concern, as the I/O circuits are often the last chance to keep unwanted currents from leaving the board.

There are several parameters of concern — internal/external, digital, analog, relays, and contact closures. They each present different problems, and may require different solutions. One size does not fit all.

Internal/external — I/O circuits connected to external traces/cables are the primary concern. I/O circuits are connected to wiring or traces that leave the board, and that can act as “hidden antennas.” If the system is shielded, internal I/O is less of a concern. If not shielded, however, both internal and external I/O deserve EMI attention. Regardless, we like to check out both types of I/O.

Digital inputs/outputs — The key concern for digital interfaces is ESD. A high level discharge may cause damage, while lower levels may simply cause upsets. The solutions include transient protection (must be fast enough), filtering, or even software (ACK/NACK protocols, etc.)

A secondary concern is radiated emissions, with small currents sneaking out the I/O port. The resulting voltages are usually so small you can't see them with an oscilloscope. Radiated susceptibility is rare with digital I/O, although we have seen problems at very high RF levels. The solutions for both radiated problems include filtering at the interface and/or or shielding of external cables.    

Analog inputs/outputs — The key concern for analog interfaces is RF. High RF levels can cause rectification in the I/O circuits, resulting in either a DC offset (no modulation) or a low frequency AC signal (with modulation.) If the modulation is in the signal passband, you can no longer filter it. Thus, the best strategy is to prevent rectification from occurring in the first place.

Typical solutions include high frequency filters and/or shielding of the external cables. If diodes are included for ESD protection, the filtering must precede the diodes to prevent rectification at the protection diodes.

Relay outputs — Since relay drivers are usually digital, the regular digital concerns apply. In addition, inductive transients from the relay coils may pose a self-compatibility problem. Snubber circuits may be needed at either the relay (best) or at the driving circuit on the boards.

Contact inputs — Since the receiving circuits are usually digital, the regular digital concerns apply.  Do NOT assume that because contacts are relatively slow that they are immune to EMI problems.  Remember, high frequency EMI currents love to exploit unprotected low frequency ports.

To recap , when designing or reviewing circuit boards for EMI, ALL of the  I/O circuits deserve  EMI attention! Even one unprotected I/O port can wreak EMI havoc.


Previous entries in the series

EMC Basics #1: Welcome!; and Clocks: critical circuits for EMC

EMC Basics #2: Resets as Critical Circuits

EMC Basics #3: Voltage regulators as critical circuits

EMC Basics #4: Analog devices as critical circuits

About the author

Daryl Gerke , an EMI/EMC consultant since 1987, along with business partner Bill Kimmel, focuses on design and troubleshooting (not test and regulations). He and Kimmel have been chasing EMI problems for over 80 years (combined, of course.) He is a published author and columnist, and their EDN Designer's Guide to EMC (1994) is still in relevant and in demand. He can be reached via http://www.emiguru.com or his other blog at http://www.jumptoconsulting.com/.

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