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Old 7th August 2012, 05:23 PM   #111
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Are these values correct? -- as the voltage divider after the reference U3 knocks the voltage down by 90%.
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Old 7th August 2012, 05:35 PM   #112
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Quote:
Originally Posted by jackinnj View Post
Are these values correct? -- as the voltage divider after the reference U3 knocks the voltage down by 90%.
Yes. values are correct. The voltage coming from the TL431 is divided by 10 to reduce the noise of the voltage reference, then there is a LPF with a very low corner frequency to reduce noise again.
Finally the voltage of the reference is about 1V, with a very low noise.
The TL431 is not just a low noise voltage reference, so I used these tecniques to reduce drastically its noise.

Andrea
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Old 7th August 2012, 07:46 PM   #113
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Seems like a lot of work to derive the reference voltage is my point -- you could use an LM4041-1.2 which is a $0.334 part. Even a button cell would be less expensive, low noise, take up less space and last several years!

By the way, the standard TL431 model (from TI) doesn't contain the reactive elements. When I simulate the circuit I get a smoothly rising Z-curve, but it's not flat.

You might want to take the curve out to 20MHz and see the effect of not connecting the compensation pin on the AD797,
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Old 7th August 2012, 11:32 PM   #114
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Quote:
Originally Posted by jackinnj View Post
Seems like a lot of work to derive the reference voltage is my point -- you could use an LM4041-1.2 which is a $0.334 part. Even a button cell would be less expensive, low noise, take up less space and last several years!

By the way, the standard TL431 model (from TI) doesn't contain the reactive elements. When I simulate the circuit I get a smoothly rising Z-curve, but it's not flat.

You might want to take the curve out to 20MHz and see the effect of not connecting the compensation pin on the AD797,
In my simulation, adding the ESR and ESL of the output caps (Panasonic FC 3 x 390 uF / 35V 0.12 tan/d), Z rises smoothly with a peak of 2.2 mOhm around 1 MHz, then decreases again around 1.3 mOhm and remain flat up to 10G, without phase shift.
No effect removing the compensation of AD797 and probably it could be oscillate.
I think the rising of the output impedance comes from the high feedback loop around the AD797, and the output caps should act the right compensation.
BTW, I have ordered all parts to prototype the circuit to see how it will really perform. Surely it will need some adjustements.
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Old 8th August 2012, 10:06 AM   #115
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by andrea_mori View Post
.............The voltage coming from the TL431 is divided by 10 to reduce the noise of the voltage reference, then there is a LPF with a very low corner frequency to reduce noise again.
Finally the voltage of the reference is about 1V, with a very low noise.
The TL431 is not just a low noise voltage reference, so I used these tecniques to reduce drastically its noise.....
How would a CCS fed, LED compare?
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Old 8th August 2012, 05:23 PM   #116
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How would a CCS fed, LED compare?
With a CCS the voltage reference see an higher impedance.
About the LED... I prefer the voltage reference, since the TL431 is cheap, I use it also in the preregulation stage as the feedback error amplifier, it has an adjustment pin suitable for fine voltage setting with low current flowing in the trimmer resistor, less drift.
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Old 16th September 2012, 11:22 PM   #117
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A simplified version of the regulator, max 600 mA, suitable for clock circuit at 3.3V and 5V, just built on the fly with the breadboard
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Old 20th September 2012, 10:17 AM   #118
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Are you sure that schematic is right? The common mode input range of the AD797 is about +-3 to 3.5V with a +-6V supply or 6+- 3 to 3.5V with a 12V supply. I don’t think you will be able to compare voltages that close to ground with that op-amp.
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Old 20th September 2012, 10:47 AM   #119
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Are you sure that schematic is right? The common mode input range of the AD797 is about +-3 to 3.5V with a +-6V supply or 6+- 3 to 3.5V with a 12V supply. I donít think you will be able to compare voltages that close to ground with that op-amp.
You're right, in fact I measured 1.34V at non inverting input of the op amp because some current flows through op amp input to LPF.

I have changed the network divider to set voltage reference to 2.5V and so seems to work properly.
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Old 31st December 2012, 01:22 AM   #120
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The breadboard seems to confirm the simulation: very stable, no oscillation, noise under the noise floor of my equipment.
Follows the breadboard final schematic and simulations already posted.
Before making a PCB prototype I simulated some popular regulators, follows my results, with parasitics ESL and ESR of used capacitors.
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File Type: jpg TWRPS_2 .jpg (75.2 KB, 55 views)
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