Mike, In answer to your last question, I believe the lowest-inductance option involves capacitors placed directly between the two power planes (and not through the ground plane). That said, the scenario scares me. For many components that require multiple planes (e.g., FPGAs w/ SerDes), some of their supplies are very sensitive to noise. For example, nearly all SerDes cores have an analog supply for phase-locked loops (PLLs), and noise injected into them can severely degrade jitter performance. Similarly, noise on the I/O supply of a parallel bus can degrade SI of the output waveform. In many instances, the chip supplier provides guidelines for isolating such voltages, often recommending specific isolation circuits. Adding decoupling between the various voltages will/can defeat the isolation circuits and inject noise from one plane onto the next. Although adding decoupling between the power planes will help with discontinuities of the signals traversing the split, you could be inadvertently creating other problems. As such, I suggest you look closely at the power planes that you're about to inject noise into and ensure they are immune to outside influence. Good luck, Pat Zabinski Mayo Clinic > Some diff pairs on L4 will cross power plane splits (actual different > power sources and loads) and I wanted to provide an effective AC path > for any common-mode return currents. I was thinking about placing some > nearby decoupling caps from plane-to-plane across the split. Do you > think it would be better to decouple from plane-to-ground on > both sides > to steer the current through the L6 ground layer? L5 and L6 > are already > coupled through the inter-plane capacitance (they're about > 4mils apart). > Which will provide a lower inductance path? ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu