Boris Yost wrote >> Does one really want to keep the "internal" of an IC decoupled from the I/O's at high frequencies? If the process technology permits all the voltages to be the same, is it better to lay down one low-impedance continous plane? How important is it to keep the 'internal' of an IC supplied with quiet power--especially if it is at the expense of the output drivers? If one attempts to separate "internal" from "output" or "specialty I/O" from "internal", will signals crossing the split within the IC then make lots of noise because their return currents must go around a circle to be completed? <<
No matter how sophisticated the IC, its output current comes from a power source off-chip, and must return -- via the board and the chip -- to that source. Failure to insure current goes where we want it to, does there what we want it to, and flows, when its work is done, where it has to go, is probably the single most overlooked principle of EMC design. What we call decoupling is often insufficient to supply the currents needed, when they are needed, with the result that the isolation someone has worked hard to provide ... makes things worse. One of the questions I used to ask applicants -- when I had a job myself -- was, "How much capacitance is needed on the power pins of (this) device?" It turns out to be a neat little problem which many designers ignore by lifting values from application notes or brassboard test circuits. Sometimes they're lucky. When we use devices with multiple power pins, we have to be sure we can provide the current each pin needs, and when it's needed, too -- at THAT pin. This pretty much reprises the old EMI hope (because we rarely GOT it) of at least one bypass capacitor on each Vcc pin. But I'd think it becomes more critical for large, high speed devices because current must flow in pins supporting state changes as they occur across the device, and failure to insure a low-impedance source and return for EVERY Vcc pin can result in transients on the device that might cause functional problems, and EMI on the I/O pins, too. Cheers, Cortland Richmond This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ 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] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
