Re: [SI-LIST] : D/A and A/D Connections

Dennis Tomlinson (det@tellabs.com)
Thu, 22 Oct 1998 10:36:37 -0500

Hi Elya,

I'd like to chime in on the side of those who prefer the solid ground
approach. It's been my experience that any attempts to isolate an aggressor
from a victim by hacking up power/ground planes have been met with less than
stellar results. Increasing planar impedance on high speed, high current switching
devices seems to open a Pandora's box of other problems. I have, on the other hand,
had some success with the opposite tact - isolating a victim from an aggressor.
These successes are limited to cases where the victim is small; has small power
requirements; generates no frequency components above a few hundred kHz; and can
be physically located in a far off corner of the PCB. In these cases, hacking out
a small percentage of the ground in an isolated corner can enhance the isolation
without significant increase in planar impedance.

As for your question about harm due to circulating currents, this problem is
best addressed by stringent placement and routing guidelines. To form these
guidelines, simply remember that return currents follow a path of least inductance,
small current loops generate less EMI than large loops, and (as a mentor once taught
me) distance makes the heart grow fonder. (a.k.a. Physically isolate potential
aggressors from potential victims). Note: I must also add that isolation and
transmission/termination requirements sometimes work against one another.
But then, when did we not have tradeoffs to deal with?

Best of luck. Awaiting other responses,

Dennis

Elya B. Joffe wrote:
>
> Dear SI-List Members,
>
> I like to express my gratitude to those of you who provided new
> information and insight related to A/D and D/A Connections.
>
> I guess, I am still left with the dillemma of "what to do when the System (not
> the PCB) incorporates several such devices. Doesn't circulating current
> between the circuits cause any harm? Should we not decouple (or actually
> isolate (in RF, usnig Ferrites or Chokes) the various ground connections?
>
> Hope to get some insight on this too...
>
> Best Regards,
>
> Elya B. Joffe
>
> The secret of happiness is to count your blessings while others are adding up their troubles
>

And, angels must always beware the feather collector.

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****