3.15 Susceptibility

There are often applications where logic structures must be placed in hostile electrical environments. A hostile environment might be the proximity to high current switching. ESD is certainly a big concern for many applications. In large systems, the issue of lightning protection must be considered. The solution to protection will vary depending on whether the logic is to operate without error or simply survive the interference.

Interference coupling is always field coupling. Different interference types have energy associated with different parts of the frequency spectrum. For this reason, different approaches are needed for each type of interference. A lightning pulse is associated with a 100,000 A with rise-time frequency of 640 kHz. An ESD pulse is 5 A with a rise time frequency of 300 MHz.

The control of interference coupling is almost always associated with loop area control. This is the same control used in designing logic structures. The loop areas associated with on-board logic transmission are small. This means that the transmission lines will not usually couple to external fields. The problem of greatest concern is coupling to cables that interconnect parts of logic systems. The subject of interference control is treated in the author's book “Grounding and Shielding and Interference Control,” listed in the bibliography at the end of this book.