[BeaverRat]

Hello guys,
So for a Christmas break project I decided to build a DAC. The USB receiver is a PCM2706 and the DAC is a PCM5102A. I believe I have finished the schematic and the first draft of the PCB design, but I would love some feedback on the schematic and board file. Are there any issues that I should fix, any problems in the schematic?
Thanks
 

 

 

[tomb]

Just some quick impressions - nothing more detailed than that, and I far from being any sort of expert. 

 
1. You may need some sort of power capacitor - a large electrolytic, typically about 1000uf or higher to stabilize the power connection through USB.  It's one of the issues with the ODAC, IMHO, in that it has no power cap anywhere on the PCB.  If USB power is a little "iffy" or unstable, chances are that it gets knocked offline more often than not.  A power cap is insurance.
 
2. I think you need some more ferrites in there - perhaps one after the main USB 5V buss, and at least another ahead of the input to the regulator.  You have one ahead of the DAC supply voltage, but there may be another place or two where they might be useful.  USB power is dirty.  Ferrites at strategic places insure that high-frequency trash (above audio) doesn't screw up the digital/analog processing.
 
3. Many people are going straight to a clock oscillator chip these days.  The crystal is fully functional as you have it, but you may get better performance with fewer parts and less cost with a clock oscillator, instead.
 
4. As I say, I haven't studied it thoroughly, but you may want a separate, dedicated regulator for the DAC chip in addition to the USB chip.  Sharing the supply and current between both of them may not result in the best performance.  However, you need to temper this with your goals to keep it simple.  If simplicity is your goal, then maybe it will work fine the way you have it shown.

 

 

[BeaverRat]

  Just some quick impressions - nothing more detailed than that, and I far from being any sort of expert. 

 
1. You may need some sort of power capacitor - a large electrolytic, typically about 1000uf or higher to stabilize the power connection through USB.  It's one of the issues with the ODAC, IMHO, in that it has no power cap anywhere on the PCB.  If USB power is a little "iffy" or unstable, chances are that it gets knocked offline more often than not.  A power cap is insurance.
 
2. I think you need some more ferrites in there - perhaps one after the main USB 5V buss, and at least another ahead of the input to the regulator.  You have one ahead of the DAC supply voltage, but there may be another place or two where they might be useful.  USB power is dirty.  Ferrites at strategic places insure that high-frequency trash (above audio) doesn't screw up the digital/analog processing.
 
3. Many people are going straight to a clock oscillator chip these days.  The crystal is fully functional as you have it, but you may get better performance with fewer parts and less cost with a clock oscillator, instead.
 
4. As I say, I haven't studied it thoroughly, but you may want a separate, dedicated regulator for the DAC chip in addition to the USB chip.  Sharing the supply and current between both of them may not result in the best performance.  However, you need to temper this with your goals to keep it simple.  If simplicity is your goal, then maybe it will work fine the way you have it shown.

 

Thanks!
 
-I added as big a cap that will fit next to the 5V line. Probably around 220uF is the max size. 
-Added two more ferrite beads, one on the 5V USB power line, and one on the 3.3V line going to the usb reciever
-For simplicity and cost, gonna try it with one reg for now
-Gonna keep the XTAL for now, I found a very precise and accurate crystal to use.
 
Anything else?
 

 

[BeaverRat]

Ok here we go - a real question. In the datasheet for the 5102A, they briefly touch on using the DAC in standard PCM mode or PLL mode. In PLL mode the SCK output from the USB receiver is not used, and the DAC instead creates its own SCK using the LRCK signal. Which of these two options is better?
 
Thanks