Audio circuit : pcb layout impact

[Sylvain Munaut]

Hello,


I have a schematics for a Audio DAC output stage.
Now, I need to do the layout. What I'd like to know is if the layout
has a big impact on quality ?

The input is a differential, coming from the DAC. The output is single ended.

What should I take care of ?

Is a different trace length gonna matter at theses frequency ? I know that
it matters a LOT for a 200Mhz signal but for audio ?

Should I use "wide" tracks ? 6mil, 12mil, more ? (The passive components are
0603 sized so I doubt I can use much more than 12mil)

I can't avoid via completly, one of the differential track has to cross the
other for one of the channel (left of right). But does that matter much ?

Should I use large, small via (what drill size ?) ...

Any tips is welcome. I'd like this circuit to be qualifiable of HiFi


Thanks,

Sylvain

 

[Joerg]

Hi Sylvain,

The main thing I'd do is provide a nice full ground plane. You don't
want any of the data noise to show up in the audio path. That would be
my biggest concern. Trace widths for the signals aren't that critical as
long as the traces don't meander through digital territory too much.
Also, provide enough bypass caps.

Regards, Joerg

http://www.analogconsultants.com

 

[Sylvain Munaut]

Hi Joerg,

The main thing I'd do is provide a nice full ground plane. You don't
want any of the data noise to show up in the audio path. That would be
my biggest concern. Trace widths for the signals aren't that critical as
long as the traces don't meander through digital territory too much.
Also, provide enough bypass caps.

Thank you very much.

I have a full common ground plane, as well as two big power trace on the
bottom for the op-amp power. 100nF Bypassing caps at each op-amp as well as
10ľF caps at each end of the power traces.

All the audio stuff is on one border of the board and the closest it gets
to digital is near the DAC, no return path for anything else in the way.


Regards,
Sylvain

 

[Ban]

Sylvain Munaut wrote:

The main thing I'd do is provide a nice full ground plane. You don't
want any of the data noise to show up in the audio path. That would
be my biggest concern. Trace widths for the signals aren't that
critical as long as the traces don't meander through digital
territory too much. Also, provide enough bypass caps.

Thank you very much.

I have a full common ground plane, as well as two big power trace on
the bottom for the op-amp power. 100nF Bypassing caps at each op-amp
as well as 10ľF caps at each end of the power traces.

All the audio stuff is on one border of the board and the closest it
gets to digital is near the DAC, no return path for anything else in
the way.

Analyze each knot separatly and determine if it is high impedance, like the
+in and -in pins of the opamps. Try to make these knots as short as
possible, do *not* route them across the board as you have written. Put
smallish Rs (33-100R) into the output lines right after the opamp.
Vias and track size do not matter. Use bipolar opamps(i.e. AD8672) not CMOS.
Route complementary signals next to each other, so they have a similar
capacitive loading. Use Bessel filters for the reconstruction filter.
Have separate isolated power supplies for the digital and analog circuitry
and use wires which connect them directly to the supply pins/ground plane of
the DAC.
Connect the digital S/PDIF input with optical(TORX173) or transformers.
--
ciao Ban
Bordighera, Italy

 

[Joerg]

Hi Sylvain,

Well, here, I don't have separate power-supply ... Every mixed signal
chip
(the ethernet PHY, video DAC, audio DAC, SPDIF TX/RX...) wants it's
separated
power supply, but that's gonna mean a lot of different power supply.

Usually that's not necessary. If you really wanted to decouple you could
use a simple resistor of a few ten Ohms or less depending on current,
and caps on both sides. That takes care of higher frequency noise.
Ferrite beads are also good but only if glued down so they won't rattle
or disintegrate.

Regards, Joerg

http://www.analogconsultants.com