I'll throw in my two pennies' worth here. Split and isolated ground planes have quite often caused me, as the guy who had to fix (and got blamed for) emissions, a lot of trouble, even though engineers who insisted on using them swore the opposite should have been true. They were right; the opposite SHOULD have been true.
What did them in was thinking that merely isolating ground and power planes would do the job. That is not enough; one must also insure ALL relevant current returns only to the isolated circuit -- and keep it from coupling or radiating elsewhere when you do that. Hardly anyone does. A useful trick is to treat an isolated circuit as if it were on a separate board, or even meters away in a different cabinet. This forces attention to current flow which may otherwise be lost three days from layout deadline. In addition, isolated sources of noise must be scrupulously attended to. An isolated digital device cannot draw on capacitance elsewhere on a board; it must be supplied locally all the switching current needed, and fast enough to keep excessive power/ground switching transients from appearing on signals leaving the isolated area. One sure sign this has not been done is when EMI goes DOWN after replacing with a zero-ohm chip the ferrite bead isolating a power island. Cortland This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
