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separating digital ground from analog ground

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Junus2012

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Hello,

What is the advantage of separating the analog ground from the digital ground in the IC design when in the PCB I am going to connect them to the same ground potential? I mean by end they will be tied together.

Thanks

Best Regards
 

Just as you never get a "perfect" short you never get
"perfect" isolation. You can however affect those values
by how you wire and how you lay out those "wires".

The main interest is probably in making sure all the
digital edge current impulses (as-smeared by local
decoupling) take a different current return path than
analog signals' return (and that you segregate by effect,
not by what somebody called them). You might have
a switched power resource or output from an analog
section, which would really ought to be tied to the dirty
digital ground (or a third, if perhaps you now have an
intermittent amp of pulsed load which the digital side
could do without).

Think in terms of the current loops, individual and aggregate,
and a device / block's sensitivity to stray transient "input
by way of sub-par ground integrity". We tend to think in
terms of voltage noise but many voltage noises are
outcomes of current-switching into branches that do
not receive the same inspection, attention (other than
when we are forced to dope out some misbehavior well
later in the development than we would have liked).
 
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