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ultra low noise power supply design

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David Hess:

--- Quote from: dom0 on August 20, 2019, 12:48:30 am ---Real® low noise regulators with decent PSRR are built by using a precision current reference and a low-noise resistor to set the output voltage, because that allows you to put just a couple µF on that node to largely kill the reference noise. This is very much unlike slapping a cap on the "ADJ" pin of old linear regulators.
--- End quote ---

It is exactly like bypassing the adjustment pin of a linear regulator except that the current reference depends on an external resistance.  The difference is that the control loop does not have to multiply the reference so it always operates at unity gain.  I tend to design my variable power supplies the same way to improve the frequency compensation; it is mostly relevant to designs with a minimum of output capacitance.

I forget where but Bob Dobkin talked about coming up with the idea for the LT3080 series of regulators long ago but that he had to wait for process technology to make their design feasible.

Update: Here.

Rerouter:
Its not too hard to do forward analysis of the noise sources of simple circuits to figure out exactly how noisy things will be for you, having circuits self cancel is always nice, but not always practical, and in every op amp feedback loop, the noise gain is always the same as non inverting gain, e.g. greater than 1,

I've recently found calculating noise over frequency bins is much closer to reality than just static "I have this much bandwidth, so I have this much noise" because the filter elements in your circuit can shape things in your favor quite a lot. and every RC filter acts like an impedance divider for the noise (think resistive divider for a given frequency)

you may find an op amp and a voltage reference may allow you to make a much lower noise power supply as a post regulator.

BU508A:
Maybe you'll find this article interesting:

https://people.phys.ethz.ch/~pmaerki/voltage_regulator_2013_0.02/20140120a_voltage_regulator_2013_0.02_description.pdf

Comparing some PSUs (from the article):


iMo:
I think the adding ie. the LM317/337 in the control loops of the above OPA1241s would keep the noise numbers while increasing the Iout_max (plus you get the protection).
PS: not sure 100n in the opamp's feedback was the right choice..

David Hess:

--- Quote from: BU508A on August 20, 2019, 09:10:51 am ---Maybe you'll find this article interesting:

https://people.phys.ethz.ch/~pmaerki/voltage_regulator_2013_0.02/20140120a_voltage_regulator_2013_0.02_description.pdf

Comparing some PSUs (from the article):
--- End quote ---

The audio and low phase noise oscillator guys have published a lot of measurement results also.

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