gEDA-user: analog/digital partitioning

[gene glick <carzrgr8@xxxxxxxxxxxxx>]

I'm throwing this out to the list for opinions. . .

This design has mixed analog and digital circuits.  For example, there's 
stuff like fpga and uP that are clearly digital and belong in the 
digital partition.  Then there's a few mixed mode devices, DAC, ADC plus 
a couple of others.  There is an analog ground plane and a digital 
plane, connected together somewhere, and they reside on the same layer 
with a moat separating them.

One school of thought is that the analog and digital planes connect at 
one single point, like at the DAC for example.  This is not practical on 
the layout since there are multiple mixed mode parts - so can't have 
just one tie point.  If each mixed mode device allows the analog and 
digital planes to touch, slots are created in the plane which is a no-no 
for EMI stuff and of course, signal integrity.

Another school of thought is to use just one single ground plane.  As 
long as the signals are well partitioned, digital and analog currents 
won't mix on the plane.  Sounds good, but I am concerned with creating 
some quiet analog circuits and the notion of mixing in digital hash onto 
the plane makes me nervous.

Yet another is to keep the slots and allow the digital signals to cross 
into the analog region across the moat.  I doubt this is a good idea at 
all.  Even psuedo-static lines crossing the moat bugs me even though I 
can reason it out as passable.

Adding another twist, there's also high-power analog on the card.  With 
good routing, I am pretty certain the high-current runs will remain 
isolated and not interfere with the low-level stuff.  But, a single 
continuous ground plane might just allow currents to flow where I really 
don't want them to.

OK, enough babbling - any thoughts?

gene




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