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Old 21st November 2009, 05:03 PM   #551
Pano is offline Pano  United States
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OK, cool.
See the attached below. It's from the Sabre ESS data sheets for 3.3V use on the analog section of their DAC chip. Pretty simple. 3.3V in, 3.3 out.

I use basically the same in my DAC (ESS 9006) sans the offset and with different caps. Seems to work fine, very quiet. I have not used the AD797 but have tried some others. (some are not stable)

But what I don't understand is what the heck is it regulating? The only reference is the voltage at C136. Certainly the opamp can supply enough current, but it should be "dirt in, dirt out" - right?

Anyhow, it does work and seems very quiet. But I'm at a loss as to what it's really doing, other than supplying some current.
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Old 21st November 2009, 05:39 PM   #552
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Quote:
Originally Posted by panomaniac View Post
But what I don't understand is what the heck is it regulating?
The answer is simple: nothing. Garbage in - garbage out. And it's actually a little less quiet than the reference voltage DVCC, slightly filtered at the input.
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Old 21st November 2009, 06:05 PM   #553
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The answer is simple: nothing. Garbage in - garbage out. And it's actually a little less quiet than the reference voltage DVCC, slightly filtered at the input.
OTOH, it allows you to filter the DC input, and then buffer that filter with the opamp. That's what it does. I would prefer to use a quiet reference as input, like a precision 2.5V ref with the opamp having some gain, but it's the designers choice I guess.

jd
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Old 21st November 2009, 06:37 PM   #554
Pano is offline Pano  United States
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Yeah JD, that's what I was thinkin'.
Clean up the DVCC and then buffer it. The filtered DVCC is the reference.
Agreed that a better ref and a little gain makes more sense. That's probably how I'll go in the future - if I don't just switch to an LT1963.
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Old 21st November 2009, 06:45 PM   #555
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That's probably how I'll go in the future - if I don't just switch to an LT1963.
The AD797 may get very upset with that capacitive loading. And those 1.2V or 2.5V references are not at all quiet. And adding gain will certainly add output noise as well.
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Old 21st November 2009, 06:47 PM   #556
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Quote:
Originally Posted by panomaniac View Post
Yeah JD, that's what I was thinkin'.
Clean up the DVCC and then buffer it. The filtered DVCC is the reference.
Agreed that a better ref and a little gain makes more sense. That's probably how I'll go in the future - if I don't just switch to an LT1963.
Thanks!
Actually you can filter DVCC more heavy than shown, and then use for instance an LME49600 as buffer. If syn08 agrees to that

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Old 21st November 2009, 09:13 PM   #557
Pano is offline Pano  United States
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I used Panasonic ceramic caps in my prototype - 4.7uF (1206 package)
http://industrial.panasonic.com/www-...ABJ0000CE4.pdf

ESS recommends the following opamps in descending order of preference:
  • AD797
  • LME49710, LME49720, LME49740, LM4562
  • NE5534, NE5532
  • Rohm 4560

The eval board uses the NE5534, but it oscillated badly in my version. Wanted to use the LME49720, but it was out of stock. So used an Analog Devices opamp (can't remember what - where are my notes?) Worked well, plenty stable.
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Last edited by Pano; 21st November 2009 at 09:19 PM.
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Old 24th November 2009, 09:50 AM   #558
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P-A,
First, thanks for the reply on the other forum.
Can you tell us a little bit about the testing process?
Thanks to you and all who are helping to make this a great little supply, I look forward to working with it.

Scott
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Old 24th November 2009, 12:01 PM   #559
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I tested the regulator for PA -- so far for noise only -- the other tests to run are impedance and transient response.

A long while back, TI responded to National's recommendation on bypassing the adjust pin on the LM317 -- while it does reduce the noise it also slows down the regulator. I would imagine that if you had a circuit was subject to dramatic change in load, as for instance in some "logic" this could be an issue.

I'll note also that WJ recommended using shielded cable for the "sense" inputs of his regulator. Jan did the painstaking analysis of the regulators and the results were described in Audio Amateur in 1995.

If someone would convince Ed Dell to issue another CD-ROM with the 1990-1999 issues of AA, Speaker Builder etc. they would have my vote in the next presidential election.
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Old 24th November 2009, 12:13 PM   #560
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Originally Posted by jackinnj View Post
I'll note also that WJ recommended using shielded cable for the "sense" inputs of his regulator. Jan did the painstaking analysis of the regulators and the results were described in Audio Amateur in 1995.
Interesting. Shielding against RFI/EMI?
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