[sdiy] split ground plane

[Seb Francis]

Joe Grisso 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).
>>     
>
> Be very careful with this - some DACs and CODECs (especially from AKM)
> use different design techniques for their power and grounding.
> Depending on the design technique used for the IC, it may be sensitive
> to deltas between the analog and digital power and ground planes.
> AKM's layout notes (http://www.akm.com/AppsNotes/layout1.pdf) gives
> more details for these. If you split the ground plane for a "delta
> ground" device and induce more than 300mV of potential difference
> between the planes, the device will either freeze or (I've had this
> happen) latch up and fail. The same goes for a "delta supply" device,
> except it's the power rail that has to have a minimal delta instead of
> ground.
>   

I should also mention that I used the analog ground for both the AGND 
and DGND pins of the CODEC.  So there was no ground delta.  Perhaps this 
might inject a very small amount of noise from the digital side of the 
CODEC into the analog ground plane, but I preferred this to coupling the 
ground planes themselves at this point.

The 3 different supply pins for the CODEC were all connected to the 
analog supply, with various filtering between them.  In essence I 
treated the CODEC more as an analog device than digital and kept the 
digital supply and ground for the really noisy things like the PICs.

This ties in with the other thread about using the dsPICs with built-in 
audio DAC - it is worth remembering that uControllers are noisy beasts 
and are never going to be producing low SNR audio without using external 
converters.

Seb