[SI-LIST] Re: Return current of a trace in stripline
- From: "Mustafa Yousuf" <yousufs432@xxxxxxxxx>
- To: <bala89si@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sun, 4 Aug 2013 14:40:40 -0700
Hi Bala,
The return current is always split between the reference planes on both
sides of the trace. The farther the plane the less is the return current
flowing in that plane. From experience, in order for the return current to
flow in the closest reference plane, the other plane distance from the trace
should in the order 3-4 times as big as the distance of the closer plane.
In this case you have two issues:
1. both planes are almost at the same distance (3.7 and 4.3 mils)
from the stripline, so the return current will be split almost equally
between the two.
2. The split in the power plane will cause serious problems. The
return current will look for the path of least inductance and you don't know
where that would be. It may very well hit a critical signal far away from
your original signal and result in significant cross talk to the other
signal which may be safe otherwise.
We had serious issues in similar situation (in DDR) as you described that
caused failure of the memory. Hence you should be concerned about this case.
Thanks,
Mustafa
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Balaji G
Sent: Sunday, August 04, 2013 11:43 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Return current of a trace in stripline
Hi Experts,
We discussed a lot regarding path of return current before and this is
regarding the path of return current in a stripline trace. As far I learnt,
the return current will take the path of least resistance at low frequencies
and path of less inductance at high frequency and hence the reason that
return current travels in the plane directly under the signal's trace. My
question is if we consider a signal travelling in a stripline which is
sandwiched between the ground and split power plane where the signal to
ground distance is 3.7mils and signal to split power plane distance is
4.3mils, should we worry about the split power plane at high frequency (say
3GHz) as the signal to ground distance is the path of least inductance and
all the return current for high frequency signal trace flows in the ground
plane causing no reflection/ EMI issues? Is my thinking right? Can you
please provide your thoughts on this?
Regards,
Balaji
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