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Old 3rd October 2003, 07:50 AM   #1
Xav47 is offline Xav47  Switzerland
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Question Multiple PCM1730

Hi,

I'm actually trying to design a DAC based on TI component, and specialy pcm1730 DAC.
Why this pcm1730 ? because I could get it as sample by TI, in replacement of my previous idea (the pcm1704).

I would like to use several pcm1730 in parallel to reduce the S/N ratio, but I've a problem to make an asymetric output (based on AOP - no tube for the moment), due to the fact that the pcm1730 is using a common VREF for output (calles VCOM2), and also that I'm a newbie in the domain.

Could you give me some advise or idea? Did someone of you realized already such output stage ?

Thanks

Xavier.
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Old 3rd October 2003, 11:50 AM   #2
weissi is offline weissi  Europe
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I think PCM1730s S/N ratio is low enough, I would not think that youll hear the difference if the analog conditioning isnt the worst after the DAC...

PCM1738 offers you the possibility of mono mode where youll get about 3db better results, but it needs a C..)
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Old 3rd October 2003, 02:07 PM   #3
PsykoK is offline PsykoK  France
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Hi XAV,

you are steel surching a solution for using 2 PCM1730!

Good luck


PsykoK
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Old 3rd October 2003, 02:39 PM   #4
Xav47 is offline Xav47  Switzerland
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Yep !!!

But I think slowly that there is no real benefit to do that.

Let see....
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Old 4th October 2003, 12:11 AM   #5
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Current output DAC's parallell easily. All you have to do is verify that you set up the references correctly (of connected together, reduce the value of the sense resistor by n, if not connected, keep normal. Then of course you have to set up any op-amps in the output filter/gain stage for correct gain.

Petter
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Old 6th October 2003, 07:58 AM   #6
Xav47 is offline Xav47  Switzerland
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Hi Petter,

Thanks for your reply, but I don't understand your suggestion.

As output of the pcm1730, you have R+,R-, VCOM2, L+ and L-

If I take 2 pcm1730, I'll have 2 different VCOM2, and therefore, I'll have separeted I/V stage fo each R and L output.
Or should I forget the VCOM reference voltage ?

Do you have any picture to illustrate your idea ?

Thanks.

Xavier.
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Old 6th October 2003, 12:18 PM   #7
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VCOM2 is just a reference to enable relatively easy setting of the resulting output voltage to zero by setting up a reference at the voltage point at which the current output resides. This sets up a common mode voltage which is nulled by the output stage.

What I propose you do:

1. Have Iref resistor and short the IREF's between the two devices.
2. Short R1+ to R2+, R1- to R2- etc.
3. Use any one of the VCOM2's
4. Set up correct gain of output stage (probably half of what you have now given that you have twice the output current.

Petter
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Old 6th October 2003, 12:28 PM   #8
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What exacly is the benafit to this?

What should the theoretical benafit actuall be?

I know arcam parallels four wolfsons in thier FMJ player but why?

If this results in much better sound quality then Ill alter my design to include another two chips, for Balanced operation with two chips per channel. Im going for the best sound quality really with this so if it really is a good thing to do then I will do it. Im usin the PCM1794 BTW. Adding another two chips to the PCB wont be too hard as I can just copy and paste most of it in.

Im thinking of redesigning the whole thing anyway using another program.

Cheers for any help Matt.
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Old 6th October 2003, 12:45 PM   #9
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5th, is there a source in Europe already for the PCM1794? TI lists no stock, but they give Digikey as a source.




Paralleling outputs the way Petter described might improve S/N a bit and THD even less so.

The classic reason to parallel is that you want to cancel distortion by using both channels on one single stereo chip (because presumably they have better tracking if they are on the same chip) in mono mode , which involves feeding the left signal to chip 1, making sure the the right channel of this chip sees an inverted signal. The right signal then has to go to chip 2, again with the signal inverted for the right channel.

If the chip doesn't have a provision for mono mode built in, this involves rather complicated data separation logic.
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Old 6th October 2003, 12:48 PM   #10
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Ok well im going to use the Mono mode of the PCM1794 anyway so I guess thats not important. The distortion of the chips looks to be very low as it is so that seems even less important again. Well that makes everything easier to make then with a lower parts count.


Cheers Matt.
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