[SI-LIST] Re: Analog and Digital grounds
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 13 Sep 2005 10:38:01 -0400
One thought to add to this:
Many engineers grow up thinking "voltage," but not "current."
Then when applying the old rules (which probably made a lot of sense before
ground planes!), they got to thinking that the objective must be to keep the
digital noise *voltage* out of the analog ground.
When in reality the problem may be keeping the return currents separated, in
places where they need to be.
Many Analog/Digital ICs have separate analog and digital ground pins, but
recommend connecting them together to the same ground plane under the IC.
Sharing the same pin(s) would have allowed the currents to co-mingle, which
then causes undesirable voltage drops in the package.
Sometimes, not enough (if any) attention is paid to noticing where the
currents go. It's the currents that make electricity work.
Regards,
Andy
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