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Old 21st January 2010, 08:48 PM   #1
diyAudio Member
 
Join Date: Aug 2009
Default PCM1704 advice

I'm making a DAC for myself and am laying out the pcb at the moment. I have a couple of final questions I'm hoping the experts here can help out with:

- Does the 1704 benefit from having seperate analog/digital supplies (though grounded together)? The datasheet says it doesn't but I'm not sure whether to believe it.

- What's the best value of capacitance to use for the BPO/Servo/Ref DC caps? I've seen some boards use different values than the recommended ones, and some say to use as large caps as possible.
- Also, a sub-question, do these DC caps need to be as close as possible to the chip like the bypass caps or is it ok for them to be a little further out?

- Does anyone out there have any app notes or any other good info for this chip? I've seen a diagram or two floating around but TI won't supply me with anything since I'm just a diy hobbyist and not Denon or something

Thanks for reading.
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Old 22nd January 2010, 03:06 AM   #2
BrianL is offline BrianL  United States
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There should be some manuals for evaluation boards on the web site that contain schematics and/or board layouts. They may not be specifically for the PCM1704 -- they might be for another related component and happen to use the PCM1704 DAC. Study these
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bel
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Old 22nd January 2010, 04:00 AM   #3
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Join Date: Nov 2009
Location: Los Angeles
Quote:
Originally Posted by projectwhite View Post
I'm making a DAC for myself and am laying out the pcb at the moment. I have a couple of final questions I'm hoping the experts here can help out with:

- Does the 1704 benefit from having seperate analog/digital supplies (though grounded together)? The datasheet says it doesn't but I'm not sure whether to believe it.

- What's the best value of capacitance to use for the BPO/Servo/Ref DC caps? I've seen some boards use different values than the recommended ones, and some say to use as large caps as possible.
- Also, a sub-question, do these DC caps need to be as close as possible to the chip like the bypass caps or is it ok for them to be a little further out?

- Does anyone out there have any app notes or any other good info for this chip? I've seen a diagram or two floating around but TI won't supply me with anything since I'm just a diy hobbyist and not Denon or something

Thanks for reading.
I've used Burr Brown chips in commercial designs and found their data sheets to be excellent as are Analog Devices. If they say it's OK, I would treat it as fact unless proven otherwise. As a general rule, digital chips exhibit dynamic loads and require the bypass caps to be as close as possible and for low noise, a ground plane in a 4 layer board is much preferred to ground 'trees' threading through various noise sources. Surface mount bypasses nearly on top of the device will rarely get you in trouble. Multiple supplies aren't absolutely needed if everyhing if properly bypassed but won't cause problems if used. 'Typical applications' are always a good starting point for BB and AD chips and you may find it totally acceptable as is.

G
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Old 22nd January 2010, 04:31 AM   #4
EUVL is offline EUVL  Europe
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Join Date: Oct 2003
Have a look at the implementation from Nagai-san of Denon :

Tefron PCB using VISHAY Z-Foil Registers

Patrick
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Old 22nd January 2010, 04:31 AM   #5
diyAudio Member
 
Join Date: Nov 2008
Location: Australia
Hello, you may benefit by looking at the thread
Silicon Chip DAC kit
as this can be used accross BB/TI Dacs, we have endevoured to explore all possibilities for improvement at a conservative outlay
Alfred

PS and also
http://rockgrotto.proboards.com/inde...ad=4477&page=1

Quote:
Originally Posted by projectwhite View Post
I'm making a DAC for myself and am laying out the pcb at the moment. I have a couple of final questions I'm hoping the experts here can help out with:

- Does the 1704 benefit from having seperate analog/digital supplies (though grounded together)? The datasheet says it doesn't but I'm not sure whether to believe it.

- What's the best value of capacitance to use for the BPO/Servo/Ref DC caps? I've seen some boards use different values than the recommended ones, and some say to use as large caps as possible.
- Also, a sub-question, do these DC caps need to be as close as possible to the chip like the bypass caps or is it ok for them to be a little further out?

- Does anyone out there have any app notes or any other good info for this chip? I've seen a diagram or two floating around but TI won't supply me with anything since I'm just a diy hobbyist and not Denon or something

Thanks for reading.

Last edited by alfredrofe; 22nd January 2010 at 04:48 AM.
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Old 22nd January 2010, 05:30 AM   #6
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Join Date: Aug 2009
Thanks for the replies.

I have the eval kit datasheet for the 1704, it's been quite helpful. The thing is that it isn't stated anywhere whether the specs in the 1704 datasheet are actually achieved by the eval board. I doubt that double layer board gets -120dB noise, and that puts the other specs into doubt also. So really I don't know whether I should follow it and risk getting only "sufficient" specs.

I saw that teflon board earlier as well and it raised more questions that it gave answers, unfortunately. For example, why does he use much larger size caps? Why metal-film bypass instead of ceramic when ceramic has lower impedance AFAIK? Also, he uses lower impedance reservoir caps, which would cause a much larger anti-resonance impedance in parallel with the smaller bypass caps. I wish there were some measurements to confirm the performance of the board.

BTW I hope it doesn't come across like I'm whining here, because I'm actually really enjoying all of this
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