PCM2902 analog--digital grounds

jmar

Member
2004-01-07 1:39 pm
Indiana
I see in many schematics for this part that the analog and digital grounds are seperated by a 10 ohm resistor.

I thought instead on my board that I would try to create (if possible) isolated traces on all analog and digital connections with every trace grounded at the filter cap ground.

Wouldn't that be a superior method....and could I eliminate the 10 ohm resistor?

THANKS.

--Mike
 

kevinkr

Administrator
Paid Member
I wouldn't recommend this due to the inductance of the individual traces. I would read the application notes and see if they provide any general guidelines.

Conventional practice with DACs has been to have a digital ground plane and an analog ground plane that meet at just one point under the dac, and this seems like pretty good practice for a 2902 as well.

In some cases one very solid ground plane has been used with good results as well.

Kevin
 

jmar

Member
2004-01-07 1:39 pm
Indiana
So the resistor isn't needed?

I thought the star system of dedicated traces and a SINGLE grounding point was ideal in any system?

I need to learn more about this......a large (low inductance) ground plane vs the star system.

Perhaps I should do the analog grounds in a star and use a single ground plane for the digital with everything going to the chip ground?
 
jmar said:
So the resistor isn't needed?

I need to learn more about this......a large (low inductance) ground plane vs the star system.


the resistor is a potential danger

I put my thoughts on paper, long ago

http://www.tentlabs.com/Info/Articles/Supply_decoupling.pdf

It may help you in learning about the subject

Again, don't trust semiconductors manufacturers app notes, I once improved a demo board using my philiosophy: The jitter decreased by a fair amount and noise went down 30 dB........

cheers
 
jmar said:
Is there an advantage at all to having (completely) seperate power supplys?

One for the analog DC....the other for the digital?

With completely isolated ground planes for each?


Hi

Yes, crosstalk should be avoided, so seperating power supply circuits and decoupling helps

On the groundplanes: Did you finish my article yet ?

best regards
 
Hello!

My experience when designing a circuit with the PCM2906, was that, i simply didnt connect analog in/out grounds with the digital grounds (USB gnd) at all! ... instead, the analog grounds are taken from the external equipment to which you are going to connect the interface to. The digital grounds however, are connected to the USB ground. This way, i found that it had 0 noise. I checked it on a pair of professional studio monitors, with absolutelly silent output when nothing was being played.

just my bit :)