Hello Everyone, I read this letter some time ago and have lots of queries regarding return currents on different stackups http://www.sigcon.com/Pubs/news/3_7.htm Consider a 4 layer stack up of this kind S1 - P - G - S2 S1 and S2 are Signal layers P is the power plane G is the Ground plane To repeat what was said When the driver switches HI the return current (transient and DC) must re-enter the driver through its power pin. When the driver switches LO the return current must re-enter the driver through its ground pin. For the given 4 layer stackup shown above...and for output of a driver (on S1 layer) switching low the return current must be on G layer i.e. layer 3 Right ? Now since there no Ground plane near Signal layer 1 It is said that the interplane capacitance between P - G layer helps to achieve this thing. i.e. P-G capacitance helps the output signal on signal layer 1 i.e S1 to get to the nearest ground plane and back to the driver ground. My question is why is S1 layer not connected to the ground plane directly through a VIA or some connection which can serve as a return path..Why do we have to depend on THIS INTERPLANE CAPACITANCE. Is this question related to Ground bounce? i'm not sure...very confused actually One more question How does ??( EM wave not electrons right ?) flow through the stack up shown below S1 - P - G - S2. Is it that EM wave flows inbetween the layers or How is it ? Plz can anyone clarify my doubts ? One more thing How does a signal flow through a VIA ? Thanks in advance, Bipin ------------------------------------------------------------------ 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.org 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