[SI-LIST] Re: Return current and stack up
- From: Cosmin Iorga <ci249534@xxxxxxxxx>
- To: jhasson@xxxxxxxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Thu, 11 Jun 2009 09:55:12 -0700 (PDT)
JF Hasson,
The return current will not flow through "any" VCC2 and VCC1 decoupling
capacitors on the board. The return current will try to flow as close as it
can to the forward path. Assuming
proper VCC2 decoupling very close to the driver device, for Layer 1 trace the
return current will try to flow on VCC1 Layer2 plane right underneath the
trace. For this to happen there should be a VCC1
decoupling capacitor placed very close to the driver device, and another one
close to the signal via to Layer5. If
you donâ??t have these capacitors, the return current will flow mainly through
Layer3 GND and will only create dynamic charge displacement in Layer2
underneath the
forward trace. The characteristic impedance in this case will be higher than
the one calculated when using Layer2
as reference plane. Simulation tools that
assign power planes as "ac" ground (most do that) will not capture this
problem.
Cosmin Iorga,
NoiseCoupling.com
http://www.noisecoupling.com
________________________________
From: "jhasson@xxxxxxxxxxxxxxxxxxx" <jhasson@xxxxxxxxxxxxxxxxxxxx>
To: si-list@xxxxxxxxxxxxx
Sent: Thursday, June 11, 2009 8:31:05 AM
Subject: [SI-LIST] Return current and stack up
Hi,
Asumming I have the following 8 layer stack up :
Layer 1 - - - - - - - Sig
Layer 2 - - - - - - - VCC1,VCC3
Layer 3 - - - - - - - GND
Layer 4 - - - - - - - Sig H
Layer 5 - - - - - - - Sig V
Layer 6 - - - - - - - GND
Layer 7 - - - - - - - VCC2,VCC4
Layer 8 - - - - - - - Sig
What would be the return path of a signal going from layer 1 (for 2 inches
running above VCC1 for instance) to layer 5 (for 4 inches) driven by a
component powered by VCC2 if the component is set up above layer 1 and
driving high ? Would it be going through decoupling capacitors of VCC2 to
decoupling capacitors of VCC1 to then get to the layer 6 reference plane ?
Best regards,
JF Hasson
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