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Old 4th February 2012, 09:45 PM   #20351
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Quote:
Originally Posted by SY View Post
And some lower voltage ones may have lower noise than some higher voltage ones. So what?
The 'so what' is that one must choose a low noise reference based on construction, as well as dc voltage.

Joshua suggested that voltage is not the only consideration, and he is right.
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Old 4th February 2012, 10:03 PM   #20352
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… Again, I'm not following your point…
So be it.
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Old 4th February 2012, 10:05 PM   #20353
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Originally Posted by janneman View Post
That is normally not the case. …
It may be the case. See Rod's reply.
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Old 4th February 2012, 10:13 PM   #20354
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Originally Posted by SY View Post
And some lower voltage ones may have lower noise than some higher voltage ones. So what?
So that a certain higher voltage reference may have a lower noise than the total noise of certain other lower voltage references in series.

Your initial statement is true for certain specific cases, it isn't true for all cases.
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Old 4th February 2012, 10:27 PM   #20355
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Quote:
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Thanks, John. What's the current that the supply you're talking about has to deliver?
That's not the interesting question. The interesting question is: Where is the money?
oohps, no, not here: on what DC voltage?
More current is easy. Just parallel enough of these thingies.

1 nV/sqrt Hz on a 100 mV source is beginner level. Divide what you have and conquer
if you have something halfway reasonable.
(exaggerating.. 100mV is pathetic.)

1 nV/sqrt Hz on a 10V source requires some thinking, but can be done for less than
€25 on Digi-Key parts. Just remember that 4 AD797s give you 500pV/sqrt Hz and that
that bounds your loop error budget. Spend the rest on your reference. A LT6655
should be a good start, still needs some filtering. Or a string of red LEDs.
OK, make that two ADA4898-2s for the amplifier to meet the cost limit.

If you need more than 15V for your line level input stage, rethink your concept.
Or prepare to buy fat capacitors.

regards, Gerhard
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Old 4th February 2012, 10:43 PM   #20356
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Gerhard, you have NO idea of what I have to do to make a quiet power supply, THAT ALSO sounds good.
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Old 4th February 2012, 11:02 PM   #20357
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Quote:
Originally Posted by Joshua_G View Post

Your initial statement is true for certain specific cases, it isn't true for all cases.
No, it is true for all cases. It doesn't matter if it's a stack of red LEDs, a stack of TL431s, a stack of 12V zeners, whatever you choose. Stacking increases s/n. Period.

You can't wave away physical first principles.
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Old 4th February 2012, 11:03 PM   #20358
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I bet lots of us would be interested in your assessment, like a relative weighting, of the importances of low vs. resistive source impedance from the power supplies to the signal pass stages. Could also be described as optimum bandwidth of the power supply. You've given some practical clues, but a more formal overview would be welcome, if possible.

Thanks very much,
Chris
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Old 4th February 2012, 11:05 PM   #20359
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Quote:
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Just remember that 4 AD797s give you 500pV/sqrt Hz and that that bounds your loop error budget.
That's the input-referred noise voltage density spec. Will that be your output noise?

I would assume that you'd double the input current noise density as well, which will make the reference slightly trickier.
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Old 4th February 2012, 11:07 PM   #20360
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Quote:
Originally Posted by SY View Post
No, it is true for all cases. It doesn't matter if it's a stack of red LEDs, a stack of TL431s, a stack of 12V zeners, whatever you choose. Stacking increases s/n. Period.

You can't wave away physical first principles.
There are some single higher voltage reference devices that are quieter than other lower voltage reference devices stacked.

Physical principles don't contradict some devices being inherently quieter than others, even quieter than other devices stacked.
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