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Old 3rd January 2012, 11:40 PM   #841
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Quote:
Originally Posted by 1audio View Post

I have serious mixed feelings about using LED's as references. They will have the following issues; temperature sensitivity with a significant tempco, noise and optical sensitivity. I got burned in the past with LED's being the source of hum in a circuit. Others disagree. I need to get some representative LED's and test them to see if they have changed. However getting a stabilized bandgap with a suitable amp in a TL431 for less than $1.00 it a great deal.
LED reference (according to some) has lower noise than bandgap reference. i suppose it depends on the particular LED as well (I read one post with noise test results of various LED's forward biased at various current levels, but I can't find the post now). Long term drift and tempco are not issues for this application.

I suppose this will be relative easy to settle in our case - do a prototype with options for LED or TL431 and test the resulting performance

Alex
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Old 4th January 2012, 12:09 AM   #842
1audio is offline 1audio  United States
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Better:
I have a Quan-tech Zener analyzer. I'll check some and see what I learn. Here is an interesting introduction to the issues:Re: Pete Lefferts LED current source

On a precision oscillator, especially one with a voltage trim, supply variations translate into frequency shifts so absolute voltage can be important.

On the series regulator with the 470 uF cap the noise is the junction of the single transistor, which is its strength. A 1K/470 uF filter will move the noise of the external reference down a few dB (over 60 at 1 KHz). This makes reference noise less of an issue. Doesn't help the transient response much however.
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Last edited by 1audio; 4th January 2012 at 12:21 AM.
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Old 4th January 2012, 07:40 AM   #843
rsdio is offline rsdio  United States
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Here's a wild shot in the dark: If LED references suffer from optical sensitivity, then how feasible would it be to use the LED inside an optoisolator as the reference? Optoisolator are necessarily protected against external light input, so it would seem that this feature would effectively eliminate any optical sensitivity. They might even present less variance than a random LED. The obvious downside is that optoisolators are larger and more expensive than a discrete LED.
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Old 4th January 2012, 08:19 AM   #844
borges is offline borges  Norway
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I did a TL431 search a little while ago. ONsemi's component came out with the least noise. Their SO8 package was the one most convenient for my use.

BÝrge
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Old 4th January 2012, 03:18 PM   #845
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I would like you see the circuit simulation sent from me.

You can see an improvement in impedance (30 mOh. to 150 mOh) that can make alternative circuit interesting.
Click the image to open in full size.

It also means less noise simulated(0.2 to 0.85 pV/Hz1/2)
Click the image to open in full size.

The noise rejection is lower because it takes a reference to the entrance but not much less.
Click the image to open in full size.

You do not like LED reference, Salas supply is the lowest noise power I have measured and is regulated by LED. LT431 is a OPA and they have Possible instability. I see in the simulations unstable between 10 and 50 Hz depending on the capacitor between the cathode and reference. This Possible instability is indicated in the graph 15 in the datasheed.
Now I have designed one PCB and I will design one another.
I can prove that they work.
Greetings.

Last edited by oneoclock; 4th January 2012 at 03:22 PM.
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Old 4th January 2012, 04:07 PM   #846
1audio is offline 1audio  United States
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Oneoclock:
Be careful of simulations. They can give you answers that don't exist in the real world.
You are passing a lot more current through Q5 than I am through Q1. This can actually lead to higher noise. You need to study the data sheet.

Here is a short description of the design and background. The noise at the output will never be less than the B-E noise of Q1 (Q5) and that won't be less than about .6 nV/rtHz. You also need to be sure that Q1 doesn't saturate or the noise will be much higher and the base current will go up.

The RC network to the base of Q1 is to attenuate the external noise and make the voltage at the base of Q1 essentially noiseless. The TL431 is used as a "servo' to correct for the secondary voltage "disturbers"; load and temperature. When I tried it without that feedback, between the transistor to transistor variations and the load/temperature effects, I was getting as much as 20% variation in the output voltage. Remember, at 3.3V .3V is 10%. With 20 dB of attenuation the noise contribution to the output at 10 Hz is small. At 1 KHz is non-existent.

I have built a number of these so far and with different loads have seen no evidence of instability.

Measuring the output noise is a real challenge. The lowest noise preamp circuit without a transformer will have an input noise in the .5 nV to .8 nV range. That plus this circuit leave a lowest possible reading of 1 nV and a lot of uncertainty. I used a transformer coupled preamp to lower that internal noise floor but still have problems with hum pickup I have not been able to resolve. All that will be examined after CES.
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Old 4th January 2012, 05:28 PM   #847
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I have to give thanks for all yours explanations.

I have already designed plates and TL431 arrives today.
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Old 4th January 2012, 07:44 PM   #848
borges is offline borges  Norway
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Demian, could you please indicate which schematic you're referring to?

BÝrge
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Old 4th January 2012, 09:36 PM   #849
Turbon is online now Turbon  Sweden
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Quote:
Originally Posted by rsdio View Post
Here's a wild shot in the dark: If LED references suffer from optical sensitivity, then how feasible would it be to use the LED inside an optoisolator as the reference? Optoisolator are necessarily protected against external light input, so it would seem that this feature would effectively eliminate any optical sensitivity. They might even present less variance than a random LED. The obvious downside is that optoisolators are larger and more expensive than a discrete LED.
Why not just dip the LEDs in a jar with black paint? Brgds
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Old 4th January 2012, 10:57 PM   #850
1audio is offline 1audio  United States
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Borge:
Its the composite one that Oneoclock posted.
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