[sdiy] split ground plane

[ASSI]

On Montag 07 Juli 2008, Seb Francis wrote:
> I don't actually
> agree with what Achim says about connecting the ground planes
> at the DAC - I think they need to be connected so current
> to/from the digital stuff never flows past any analog parts
> (hence connect only back at the PSU).

You need to make sure that you have the same ground potential on 
both sides of the DAC, otherwise the accuracy of the DAC is no 
better than whatever difference is present.  So this is actually 
a 0V reference that could in principle be servoed, but then you 
have the obvious offset, frequency response and noise problems 
from the servo circuit - using a choke to keep any non-DC out of 
the picture is a good way of ensuring that there's no potential 
difference in the first place and hence no current through the 
connection.  It is then relatively easy to ensure that the return 
from this point on the analog side never makes it past any 
sensitive analog nodes.  If you can't do that or don't want to do 
that, you need to go differential.  This would be highly 
impractical for a (discrete?) R2R DAC however and the difference 
in performance is likely swamped by the errors in other parts of 
the system anyway.


Achim.
-- 
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