Carey, I thought of another likely coupling mechanism. You have a gnd split to isolate the two sections. You've probably done the due diligence of connecting the gnds at where your critical signal path crosses the realms. It's really easy for an engineer to say, "It's only <n> Mhz, non- critical, just let it cross the split and keep it away from the sensitive things on the analog side. Like most of us, including myself until I took an EMC course, we look at the line shown on the schematic as complete and "self-reliant". Right down to how we all represent a driver/receiver, we concern ourselves with the signal and ignore supply and return, which are "everywhere", right? Problem is each signal has an equal return path. For speeds below 20 Khz (yes, that's "K"), we're pretty much correct. Beyond that, the return path follows the signal exactly. If a signal of over 20 Khz crosses a ground (return) split, it's return current must find a path around the split. If some portion of your critical signal path transitioned across a gnd "web", and elsewhere a digital signal crossed the split, the return current for that digital signal likely traverses the same gnd web as your critical path. BINGO, your two realms are sharing a conductor, namely gnd, and therefore coupling occurs. It's possible or even likely that webbing the gnd under the offending digital line would not be a good idea for other reasons. If so, you may be able to tolerate this wandering return current by enlarging the gnd web at your critical crossing so that the digital returns find adequate conductor without the fields co-mingling. I believe these field effects follow the rule of inverse squares, whether in Cu or dielectric (someone please correct me if I'm wrong!), so each increment of additional distance has exponential effect in isolation. From here on out the potential mechanisms I could conjure will likely take more bandwidth than they're worth. Regards, Jeff Seeger Applied CAD Knowledge Inc Chief Technical Officer Tyngsboro, MA 01879 [log in to unmask] 508 649 9800 *************************************************************************** * TechNet mail list is provided as a service by IPC using SmartList v3.05 * *************************************************************************** * To unsubscribe from this list at any time, send a message to: * * [log in to unmask] with <subject: unsubscribe> and no text. * ***************************************************************************