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Old 13th June 2004, 05:44 PM   #61
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Quote:
Originally posted by mandat
I am suprised Chris with your results; I mean TL431 measurement. It is so noisy and correlation between current and noise voltage is so high. We should carefully set the current flowing through TL431 IC.
The Zener's diodes and LEDs are better for audio purpose. Am I right?
Well, I only measured noise, so just like Jocko and someone
else said , you have to also consider the PSRR. If your design
alllows for a reasonably quiet supply to the reference , by
regulation or filtering for instance, and you use a CCS
then it may be reasonable to go by noise figures. If, on the other
hand, you use a simple circuit where you have a noise supply
and maybe use a resistor instead of a CCS then
PSRR may be a more important issue than noise. The
standard answer "it depends" always applies.

I studied noise as a separate issue, since that is inherent
in the components. The importance of good PSRR, ie. low
dynamic impedance, can to more or less extent be designed
away in a circuit. (Jocko, correct me if I'm wrong.)



Circlotron and others,

I don't think I will measure any laser diodes. There are many
components one might wish to test, but I think it is enough
for now. I treat this as a pilot
study which gives some partial answers and ask a number of
new and more specific questions that can be studied for those
who feel inclined to do so.
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Old 13th June 2004, 07:14 PM   #62
sam9 is offline sam9  United States
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If you are talking about the use in a CCS, keep in mind Self's experiment where bypassing with a 47uF cap essentially mooted PSRR concerns regardless of the biasing method. This leaves only with noise from the diode itself.
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Old 13th June 2004, 08:44 PM   #63
Steven is offline Steven  Netherlands
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Quote:
Originally posted by mandat
I am suprised Chris with your results; I mean TL431 measurement. It is so noisy and correlation between current and noise voltage is so high. We should carefully set the current flowing through TL431 IC.
The Zener's diodes and LEDs are better for audio purpose. Am I right?
The TL431 is bandgap based and bandgaps are usually more noisy than zener/normal/light emitting diodes, unless you build a bandgap with very large geometry transistors.

Steven
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Old 14th June 2004, 12:41 PM   #64
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One should also remember that the TL431 is not primarily
designed to be a low-noise reference, but a precise
shunt regulator. According to my books a bandgap reference
has a well-defined voltage and is very temperature stable
(if designed right), and the TL431 seems to score well at
both these points.
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Old 14th June 2004, 05:54 PM   #65
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Default Measurements for LED noise optimum

Since I started this thing I guess I'd better try to wrap it up, so
I have done some extra measurements on LEDs, using a single
current source with variable current. The purpose was to spot
any tendency towards an optimal current for lowest noise. The
differences in the 2-8mA range are so small that is mostly an
academic exercise to even bother about this, but since I am
an academic I was stupid enough to waste some time on it.
Obviously the sample set of types and devices is much to small
for any claims, but if one should nethertheless be bold enough
to try spotting some tendency in the measurements, there seems
most LEDs have an optimum somewhere in the 2 to 6 mA range.
Some prefer close to 2mA , some close to 6mA and some
somewhere inbetween.

Updated report attached. New measurements at the end
under heading Experiment 2.

I think I have had enough of measuring noise for now, so
don't expect any additional measurements. At least not
for voltage references.
Attached Files
File Type: txt noise_measurements_1_4.txt (14.4 KB, 2443 views)
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Old 15th June 2004, 01:34 PM   #66
EC8010 is offline EC8010  United Kingdom
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Default Cracking stuff!

Hello Christer, I'd like to thank you for taking the time and trouble to make all these measurements - they make fascinating reading.

I went through your v1.4 set of data, averaged across your five measurements for each condition, then normalised the noise compared to the forward drop to give a figure in uV/V. Looking at your LEDs, they typically fell out at around 0.16uV/V, with a very shallow noise null (only 1dB deep centred on 5mA +/-1mA). I compared across diode types, and taking the LEDs as the reference (all at 5mA unless specified):

12V 0.5W Avalanche: - 15dB
12V 1.3W Avalanche: -13dB
True Zener: +6dB
Forward biased silicon junction (1N4148 or BC549): +7dB
TL431 at 5V, with 1mA: +12dB
Transition between Zener and Avalanche: Much noisier, and very variable
TL431 at 2.5V (or 5V, with >1mA): +34dB

This is most surprising - your data indicates that the much-maligned 12V "Zener" diode is the quietest device by far.
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Old 15th June 2004, 02:47 PM   #67
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EC8010,

Although I hoped there would be some interest in my results
I would never have expected anyone to call those massive
amounts of boring data "fascinating". You must be as crazy
as I am.

Thanks for doing the normalized figures.
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Old 15th June 2004, 05:12 PM   #68
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Thanks Christer,

It sheds at least some light on the noise behaviour of several common devices used as a voltage reference. Next step would be noise versus frequency. 1/f noise starts at different places, at some devices at 100 Hz (or even lower), at others it starts at 1 kHz. Given the 20 kHz bandwidth of your measurements that doesn’t show up. Higher noise frequencies can be easily filtered away, but lower are more difficult. Anyway thanks for the great job done.

Cheers
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Old 15th June 2004, 05:35 PM   #69
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I initially intended to look at the spectrum too, but when i
relized how much time it took just to get the RMS
figures I didn't have enough patience left to do both. Not
for now at least. If anybody else volunteers to do that I
could mail the components I tested. With a few exceptions
I bought them specifically for this test and have kept them
separately to know which ones they are.
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Old 16th June 2004, 01:01 AM   #70
jam is offline jam  United States
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Default Don't stop now......

Christer,

Come on.........you can do it.

Regards,
Jam
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