[SI-LIST] Re: least inductance path(return current)

When a signal goes out on a trace on layer 1, there will be electro-magnetic
fields around the trace.  These cause return currents to flow in conductors
near the signal trace ... having nothing to do with the DC potential of
those conductors, and little to do with what those conductors eventually
connect to.

The return paths are all nearby conductors, just because they are there.
For there not to be a return current in a nearby conductor, you would need
to shield it from the electromagnetic field.

Eventually, the return currents reach boundaries, like the driver and load,
and there the currents want to complete paths by "flowing" between power and
ground layers, for example.  If the decoupling capacitance is insufficient
there, the current sees a greater impedance to completing that path.

If you remove the power plane on layer 2, the return path becomes layers 3
and 4 as well as nearby traces on layer 1.  Some return current will flow
through parallel signal conductors on layers 1 and 3, leading to crosstalk.

This is the situation at high AC frequencies.  At DC, the current takes the
path of lowest resistance, which might be the ground plane that is on the
other side of the power plane.  At DC, proximity doesn't matter.  At high
frequencies, proximity is everything.

By definition, designing a board to have transmission lines with some
characteristic impedance on layer 1, means return currents are on the plane
on layer 2, regardless of what else you use that plane for.

Regards,
Andy


----- Original Message -----
From: "karan bagga" <kbagga31@xxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Saturday, July 26, 2003 02:54 AM
Subject: [SI-LIST] least inductance path(return current)


>
> This is a very basic doubt:
>
> SI Book says that return current follows the "least inductance path"
>
> Now supposingly i have the following stack up
>
> 1) signal
>
> 2) power
>
> 3) signal
>
> 4) gnd
>
> 5) signal
>
> If i have atrack on layer 1.
>
> Supposingly the circuit loops gets completed with the gnd on layer 4.
>
> What will the return current path ?
>
> Suppose there are ByPass caps also b/w power(2) and gnd(4).
>
> "Some books says the return current will first go to power and then
through bypass
>
> to gnd". Is it correct ?
>
> Can you please tell me why this happens ?
>
> Even if my circuit does not include power(2)
>
> why would current flow through it? ( i know i am missing something, need
help)..
>
> Thanks for the explanation
>
>
>
> Regards
>
> Karan




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