[SI-LIST] Re: Analog & Digital Grounds
- From: Gene Glick <gglick@xxxxxxxxxxxx>
- Date: Wed, 14 Oct 2009 15:15:01 -0400
Scott McMorrow wrote:
> Gene
>
> Below 5 MHz all the rules about unifying analog and digital ground
> fly out the window. Below this point you have to be concerned about
> magnetic field penetration through planes. Yes all that low
> frequency "stuff" in a digital signal is important. Via-less design
> of the audio signal section is especially important to meeting your
> noise-floor requirements. Isolation of analog and digital can be
> performed with separate grounds, lots of space, and multiple shield
> planes.
>
> Have fun.
>
> Scott
>
Hi Scott.
Yet all those reference designs from A/D and D/A vendors manage such
results using only 2 layer PCB. After looking at them, I conclude that
by using ground flood on the surface layers, the return currents usually
flow adjacent to the signal traces - i.e. no mixing of currents in the
ground plane. If that's not why they work, then I don't see it.
So what happens on 4 layer design? More complex that a single port
design? I've considered isolated ground and power planes for each
section/port and then connecting the return, ground, to each area as a
trace - sort of differential audio, but singled ended at the same time
:) Is it worth it? I have no clue.
I don't have the luxury of multiple shield planes.
Hi Boris,
Differential is probably the way to go, if I could go that route. Not this
time, sadly.
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