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ultra low noise power supply design

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iMo:
Here is a design which may work. You may use an 780x in the loop as well (its noise should cancel itself in the loop). The advantage of the 317 (or 317L) is you get an over-current and over-temperature protection too.

David Hess:

--- Quote from: imo on August 19, 2019, 09:59:39 am ---Here is a design which may work. You may use an 780x in the loop as well (its noise should cancel itself in the loop). The advantage of the 317 (or 317L) is you get an over-current and over-temperature protection too.
--- End quote ---

The idea behind that design is at least 40 years old now.  You can find it in the earliest application notes for integrated regulators.

D1 should be bypassed to reduce its AC contributions to noise and frequency response.  D1 is replaced with a transistor buffer in some implementations.

Long ago I designed and built a bunch of 10 volt 1 amp "power references" using 7805s, LT1007s, and LM329 integrated zener references.  Enclosing the high noise 7805 within the low noise LT1007's control loop eliminated its noise contribution.  Pole-zero compensation was used around the relatively fast LT1007 and for lead in the output divider although I now know how to do it even better.  Load regulation was so good that it defied our ability to measure it using 7.5 and 8.5 digit voltmeters.

iMo:

--- Quote from: David Hess on August 19, 2019, 05:14:59 pm ---..Pole-zero compensation was used around the relatively fast LT1007 and for lead in the output divider although I now know how to do it even better.  Load regulation was so good that it defied our ability to measure it using 7.5 and 8.5 digit voltmeters.

--- End quote ---

Sure, it is nothing new :)
I wanted to show how the noise contribution of the rather noisy part cancels out within the control loop in the simulation.
Could you share your schematics from that 7805+LT1007+329 combo, plz?

PS: with a 10n in parallel with D1 and with "10mVpp noisy" LM317

David Hess:

--- Quote from: imo on August 19, 2019, 05:48:12 pm ---Could you share your schematics from that 7805+LT1007+329 combo, plz?
--- End quote ---

It was long ago although I do have them somewhere.  The result was just an extension of the schematic below.

The JFET was replaced with a 2N3906 emitter follower.  Buffering is not absolutely required because the operational amplifier could sink the 5 to 10 milliamps of current from the common pin of an LM109 or 7805 regulator however the extra power dissipation in the operational amplifier spoils low frequency precision.

An adjustable regulator like in your example avoids this issue by having a very low adjustment pin current but this low current also makes the level shift element noisy which is why I suggest bypassing it.  (1) Honestly when I have made this circuit using an LM317, I put a 1.2 kilohm (if not lower) resistor between the output and adjust pin to force the adjustment pin current to 1 milliamp (or higher) just for more predictable performance of the buffer and level shifter.

R2 was replaced with the pole-zero lead network.  (2) And of course there was a pole-zero feedback network from the output to inverting input of the LT1007 which was absolutely required for stability of the LT1007 which was only used for lower noise.  A slower operational amplifier will not require external compensation. (3)

(1) "Bypass" as used here just means placing a shunt capacitor across the element to lower its AC impedance.  In this case, it allows the operational amplifier to maintains low impedance at the adjustment pin at AC.  This is very common when zener diodes are used as level shifters because of their non-linear AC impedance.  My first instinct would be to use a 1 microfarad solid tantalum across it but most anything will help and the value is not critical as long as bypassing exists.

(2) At least it was a pole-zero network as laid out on the printed circuit board.  In practice, best performance required just a 220 picofarad capacitor across R2.  This gives a little boost to the phase margin for better transient response.

(3) As shown, the LM108A is *not* compensated for unity gain which would require only the 100 picofarad capacitor to ground.  Whoever designed this example was clever and took advantage of the required external compensation to add a zero to the frequency compensation using R1 for better high frequency performance.

dom0:
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.

Typically ICs with this approach are also low drop, but there is nothing inherent to the technology requiring you to use a PNP SPE. It's not particularly hard to built this with OPs, though outdoing some modern components will be (very) challenging.

--

I'm not sure why you need particularly high resolution ADCs for a milli ohm meter, because the traditional approach to this problem is to use a LF oscillator and measure the amplitude of the current. No DC, no drift problems, you can use very narrow bandwidth, so little noise. I believe some meters even went as far as using synchronous demodulation to precisely only measure the resistive component, not any inductive components that may already arise in larger wire-wounds at a couple kHz.

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--- Quote ---As far as I know, every ripple on the supply line carries through, so even when you have a linear regulator, it will give you the 5v that you need, but all the noise and ripple follow through. same would be true for the chosen voltage reference and so on.
--- End quote ---

No. This property is called power-supply rejection ratio (PSRR), which typically peaks at 100-120 Hz and rolls off for higher frequency (loop gain), for regulators. For a Zener voltage reference the PSRR is approximated by the ratio of the supply resistor / current source impedance divided by the dynamic Zener impedance. (And PSRR w.r.t. output current for active current sources).

It is also easy to filter the output of voltage references.

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