[SI-LIST] : Analog ground isolation

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From: Chris Bobek (cbobek@cadence.com)
Date: Mon Aug 14 2000 - 15:32:44 PDT


Subjects about ground isolation on this thread never fare
well, but I'll give it a shot :-)

I have a very simple case. I have a PMC card with a couple T1
interfaces on it. So, the only ground connection to the
supply is a single "digital ground" on the PMC connector. The
LIU ICs have separate analog and digital grounds. I have
multiple ground planes that are stitched together (4 planes on
a 12 layer board).

I have two options I'm considering (please let me know if
there's a better alternative)

1) My preferred option is to simply use one ground for
everything on the PMC. I've done this in the past on a
different application and it worked well. The theory is that
you have a rock solid ground plane with low inductance, so
everything will have the same reference.

2) Use one of the 4 ground planes as an analog plane, but it
will be connected to the same ground pins on the PMC. There
wouldn't be any inductor or filtering between the two planes.
That is, the two planes are shorted together at one point
(over several ground pins on the connector). I don't like
this approach as much because you're not really accomplishing
anything with respect to ground on the motherboard. Sure, the
grounds are kind of isolated on the PMC, but once they reach
the motherboard, they become one again.

I am not too keen on using inductors or ferrites to isolate
ground planes. It sounds to me like it would cause more
problems than it would solve. I do have filtering and
isolation for analog power, so I'm addressing a lot of the
issues there.

Does option 1 sound reasonable? Is this common practice for a
T1-type card?

Thanks,

Chris

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