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Old 31st July 2007, 04:13 PM   #1
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Default DC servo in DAC

Hi,

I see dc servo between the i/V and the diff/single stages in several DAC but is it possible to put it after the diff/single stage ? For the moment I don't see how ... a condensator is the only solution ?

I was thinking the I/V offsets were auto cancelling at the diff/single stage and that the only offset was the diff/single ... do I make a mistake ?

Thanks in advance
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Old 31st July 2007, 05:02 PM   #2
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Perhaps I can find a good opa for the diff/single with offset trim connections like the opa134 ?
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Old 2nd August 2007, 06:59 AM   #3
peufeu is offline peufeu  France
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Default Re: DC servo in DAC

Quote:
Originally posted by galouche
I was thinking the I/V offsets were auto cancelling at the diff/single stage and that the only offset was the diff/single ... do I make a mistake ?

Vcc/2 offset will cancel, but offsets from component tolerances will not, so a DC servo or coupling cap may be needed, depending on the actual values... Some CDs have digital offset, too !

Also, without coupling cap, or DC servo, a DAC can output a DC voltage if asked to. On a DC coupled system this would fry the woofer.

Quote:
Originally posted by galouche
Perhaps I can find a good opa for the diff/single with offset trim connections like the opa134 ?
Do not use the opamp's offset trim to compensate anything else than the opamp's own offset, or you'll imbalance the input stage and get more distortion.
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Old 2nd August 2007, 05:32 PM   #4
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Thanks peufeu for your answer I was thinking of puting a dc servo behind the (if need because the opa827 is very low) and the opa134 for trimming only its offset.

I've read all your site articles, very interesting.you seem not to like the opamp I'll make one dac with opa827/134 and one other with D1 pass to compare.

In an article in your site you say :
Quote:
Put the clock in the DAC and slave the transport
I was thinking of using only one tent clock XO for the 2 pcm 1794 and the transport (perhaps using buffer if need) Do you think it's okay ?

merci
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Old 2nd August 2007, 08:15 PM   #5
peufeu is offline peufeu  France
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Well you can only use one clock anyway, the question is where to put it ?

I vote : clock in the DAC, slave the transport to it. For sending a 10+ MHz signal, use a proper buffer with proper impedance termination.

If I were to do it again, I'd use something simple, cheap, and proven, like LVDS over twisted pair, for instance a SATA cable has 2 shielded twisted pairs, OK for sending clock and receiving SPDIF ; or a CAT5 cable has 4 pairs, perfect for sending clock and receiving I2S.
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Old 2nd August 2007, 09:33 PM   #6
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Thanks a lot for your answer, you've even answered to a question I've not asked ... the clock output ... here LVDS ...

I work in computer networks and I have all twisted data cable for exemple cat 6 with different type of shields so for the cable it's ok

For buffering I've seen very cool conditionners like lmk02000 to 03000, clock drivers, comparators ... a lot of choice ...

Clock is a difficult subject for me as I'm a newbie for this part of the dac So it's cool being helped sometimes because we read a lot of things about this subject and it's hard to take a direction.
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