[sdiy] split ground plane

[Seb Francis]

>
>> For example, I 
>> have an r2r on the digital side whose output I want to connect
>> to a level-shifting opamp on the analog side. Is it ok to just
>> route the signals between the two planes? Or does this method
>> create a loop on the return path?
>>     
>
> If the DAC is the only domain crossing, then you should connect 
> the two planes at exactly that point.  You can use a choke for 
> the connection as it should ideally just be a DC connection.  The 
> two planes should never overlap, however, as that would AC couple 
> them.  If you have them on two different layers, then you can use 
> the digital ground plane layer for routing in the analog part and 
> vice versa.
>
>   

Sorry for posting yet another link to the 4xD project (I didn't see Tom 
had already mentioned it before my last post).  Here you will find an 
example of this type of split ground plane:
http://burnit.co.uk/sdiy/index.php?page=4xd&subpage=layout

The CODEC ICs sit in the middle and the two ground planes are completely 
separate, and connected only back at the PSU (this connection isn't 
shown on the PCB layout).  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).

Seb