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Old 4th February 2012, 08:05 PM   #20341
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Quote:
Originally Posted by abraxalito View Post
There is no 'the reference' in SY's statement. He's saying to use many. Here's a thought experiment -

Say you need a 10V reference and you wanna use TL431s. I can see two ways to go. First use resistors in 3:1 ratio to multiply up the 2.5V reference to 10V, using a single TL431. Second, use 4 TL431s in series with no resistors. The DC voltage is the same but the noise will be 6dB better in the second case. In fact the DC voltage will be more accurate on average in the second case because of lack of resistor tolerance and the power of statistics.

Its possible to mitigate the noise in the first case by bypassing the upper resistor with a suitable cap to reduce the noise gain.
You are taking about 3 *identical* devices in series. Identical devices wasn't mentioned in the post I replied to. When talking about LEDs as reference voltage, possibly different LEDs have different noise and possibly a certain LED of higher voltage may have lower noise than another certain LED of lower voltage.
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Old 4th February 2012, 08:07 PM   #20342
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No, they don't have to be identical. If the noise is uncorrelated, it adds as power, not voltage.
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Old 4th February 2012, 08:11 PM   #20343
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Quote:
Originally Posted by SY View Post
No, they don't have to be identical. If the noise is uncorrelated, it adds as power, not voltage.
Yet, there may be a reference voltage device of certain voltage with lower noise than other devices of lower voltage.
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Old 4th February 2012, 08:15 PM   #20344
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So what? Noise voltage still sums as power. Two red LEDs, for example, will have similar Vf and en. And the noise is uncorrelated unless you're invoking miniature demons. So the 3dB s/n advantage is just basic physics.

Again, I'm not following your point- do you understand the difference between voltage and power summations?
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Old 4th February 2012, 08:36 PM   #20345
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Quote:
Originally Posted by SY View Post
No, they don't have to be identical. If the noise is uncorrelated, it adds as power, not voltage.
Agree. They don't even have to be three LEDs, it can be a LED, a zener and an integrated ref. Noise (uncorrelated) adds with power, the ref voltages add linearly.
Although, I think that the minimum of 3dB for a pair is approached closer if the noise sources are approximately the same.

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Old 4th February 2012, 08:41 PM   #20346
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Originally Posted by Joshua_G View Post
Yet, there may be a reference voltage device of certain voltage with lower noise than other devices of lower voltage.
That is normally not the case. Most higher voltage references are build from a basic low voltage reference (a so called 'bandgap reference') with a build-in amplifier. So the higher voltages ones have generally higher noise in proportion to the internal gain.
In such a case, a series of lower voltage (and noise) refs have less total noise.

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Old 4th February 2012, 09:31 PM   #20347
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Quote:
Originally Posted by janneman View Post
That is normally not the case. Most higher voltage references are build from a basic low voltage reference (a so called 'bandgap reference') with a build-in amplifier. So the higher voltages ones have generally higher noise in proportion to the internal gain.
In such a case, a series of lower voltage (and noise) refs have less total noise.

jan
Bandgap references are a poor choice for applications where noise is important, a quite unnecessary degradation.

Buried zener references have as much as 20x better noise performance, and this can be improved further by increasing their working current.

So, Joshua is right - a 10V buried zener, for example, will easily surpass the noise performance of a 1,2V bandgap, despite the higher dc voltage. The difference is that a high quality buried-zener is designed for low-noise, whereas a bandgap is not.

Please read the old Application Notes of Jim Williams (God bless 'is soul) in the Linear Technology AN library, for the full story.
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Old 4th February 2012, 09:37 PM   #20348
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Apples and oranges, Rod. Stack two buried zeners or two bandgaps and you still have a 3dB noise advantage compared to a single one.
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Old 4th February 2012, 09:41 PM   #20349
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Apples and oranges, Rod. Stack two buried zeners or two bandgaps and you still have a 3dB noise advantage compared to a single one.
I was not addressing the question of stacking. I am supporting Joshua's correct assertion that higher voltage references may possess lower noise than low voltage ones.

The difference is down to the construction: Bandgaps being a poor choice if you want low noise.
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Old 4th February 2012, 09:43 PM   #20350
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And some lower voltage ones may have lower noise than some higher voltage ones. So what?
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