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Cutting through the BS of low noise supply design
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ezalys:
Hey all,

I'm interested in low-frequency low noise low distortion signal acquision and generation, mostly for audio and control loop characterization. 130 dB of dynamic range up to 400 kHz let's say on a +/- 1 volt signal. I realize that given I'm even asking these questions I've got a long way to go but I've gotta start somewhere, but I do have most of the equipment to measure such things... so that's maybe a good start. What I just can't seem to cut through is how to think about low noise power supply design. Most modern high end instruments that I've torn into seem to use off-the-shelf switching power supplies like those from Meanwell. I notice a lot of audio folks still using linear power supplies... which at this point I'm convinced is totally unnecessary. I can imagine that careful design of a switching power supply from the AC mains input to the voltage fed into the LDO can reduce switching harmonics, which the LDO no longer needs to work to suppress... which is good. How then do these high end manufacturers get away with these COTS power supplies? Can you clean up the output of these COTS supplies? What are the general strategies? At what point is it necessary to design your own supply instead of filtering like hell? I have to think it's never worth it if I just see COTS supplies in T&M equipment!

What I understand is that with a linear supply you're starting with a noisy AC signal containing mostly 60 Hz plus harmonics of this from rectification plus tons of other garbage from poorly designed electronics also plugged into the wall that barfs noise back onto the line. Is it always better to step down rectify and filter this as with a linear supply in comparison to starting with the garbage that comes from a COTS supply and filtering this? These seem to me to be equally hard problems. That seems to be the comparison I have in my mind.

Second... Is there a purely filter-based way to think about power supply design? I've always understood power supply design as trying to design some sort of high pass filter with as low as possible an output impedance. I've always imagined that an SMPS is a high-pass filter that adds it's own noise (or rather, high frequency harmonic content)... with a corner somewhere around the loop frequency, and thus post-filtering components become smaller as this corner goes up, as one needs to get rid of switching content and extend the HPF as far as possible. A linear supply to me just seems to be accepting that this filter needs good rejection down to 50/60 Hz while maintaining low impedance. Furthermore, decoupling I've always seen as stiffening up this filter at higher frequencies... basically starting at a frequency corresponding to the capacitor and the source impedance (?) and ending at the self-resonant frequency of the cap and other parasitics. Is this the right picture?

E
RandallMcRee:

No need to create the wheel again--there are many long threads about low-noise power supply design over at diyaudio.

The one that stands out as being closest to your question/quest is this one:

https://www.diyaudio.com/forums/vendor-s-bazaar/297147-silentswitcher-mains-free-15v-6-5-3-3v-power.html

Jan Didden created the silent switcher project more or less in answer to, methinks, just the question you are asking.

Randall

ezalys:
I don't really see any schematics in the linked thread or any particular overarching comprehensive explanation about why designing this switcher with a switching frequency of 1 MHz and then post-regulating is a much better way to go than using passives to do the same... or even if it's possible with passives or WHY it's not possible with passives. Or using passives and then post-regulating.

Also, does this switching frequency of 1 MHz imply that the noise that's below that is cleaned up?

I've also encountered these resonant converters... which I understand as a narrow BPF driven at its center frequency. Does this filter noise above and below the center frequency? Do you now only have to deal with the noise you've introduced with the switching in addition to what's within the BPF? What I'm really lacking is an understanding of power supplies in terms of filtering and impedance.
dmills:
One NICE trick is to lock the switchers to a (harmonic) of your sample rate, that way any switcher residual aliases down to DC and just appears as a small DC offset.

Apart from the usual low noise things (pay attention to current loops) low noise with switchers really amounts to understanding that LDOs have PSRR that falls with frequency, so you really want to dump the switcher ringing before you hit the LDO.
Fortunately the carefully dimensioned L/C LPF is made for this, but watch the parasitics and you are usually better with a couple of low Q stages instead of one high Q one (that WILL in production have the series resonance right where you don't want it), typically a few ohms in parallel with the filter inductor will help to kill the Q.

One other thing about LDOs, they sometimes have horrible low frequency 1/f noise, this is worth being a little careful of.

Johnson has useful notes about LC power filtering in "High speed digital design".
RandallMcRee:

--- Quote from: ezalys on November 25, 2018, 05:47:42 pm ---Hey all,

I'm interested in low-frequency low noise low distortion signal acquision and generation, mostly for audio and control loop characterization. 130 dB of dynamic range up to 400 kHz let's say on a +/- 1 volt signal.
E

--- End quote ---

Taking a step up in abstraction, I would say that the solution to your problem of measurement has less to do with power supply design and more to do with a low-noise voltage reference for the A/D converter. At least that has been my approach, in trying to measure signals in your voltage ballpark but at the DC limit. Inspecting data sheets for A/D converters supports this as well as app notes such as DN568, Reference Filter Increases 32-Bit ADC SNR by 6dB:
https://www.analog.com/media/en/reference-design-documentation/design-notes/dn568f_web.pdf

What do you think?
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