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Buck converter that draws a constant current

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magic:

--- Quote from: sveinb on February 02, 2024, 08:48:56 pm ---active repeaters in the cable itself are also powered off the same 5V supply, so that supply is likely to have an 8 kHz ripple even if my circuit doesn't create it.
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Right, 8kHz is the microframe interval so anything doing periodic hi-speed transfers is likely to contribute.


--- Quote from: sveinb on February 02, 2024, 08:40:24 pm ---There are several indications that there is an acoustic component. The strongest indication is that the phase of the noise is different in the different microphones on the board. If it had been purely an electrical noise path, I would expect the noise to be in phase.
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Makes sense, although I wonder if other factors could explain such differences, perhaps physical orientation if it's a matter of RFI pickup?
Mind that speed of sound in solids tends to be a 4 digit number (m/s), corresponding to tens of cm wavelength at 8kHz and not much phase difference to be seen on small distances.

MLCCs would be candidate for the source of vibration.


--- Quote from: sveinb on February 02, 2024, 08:40:24 pm ---The mics are digital, and I have tested using an external supply. That didn't solve the problem (this was before I had concluded that there must be an acoustic path also)

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In such case is there even any electric component at all, if external supply made no difference?


--- Quote from: sveinb on February 02, 2024, 08:42:34 pm ---
--- Quote ---Try adding a large (1mF or more) capacitor at the USB power input? Note that if that quick test worked, to stay within inrush current limits, you'll need to add a precharge resistor and bypass MOSFET in the final design.

--- End quote ---

This was probably the first thing I tested, and it didn't eliminate the problem. However, this was before I had concluded that there was also an acoustic noise path, so this might solve the electric part of the problem.

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Did it significantly reduce supply ripple without reducing recorded noise?

sveinb:
Update: I've done more experiments and measurements and found the source of the problem. There are five MLCCs on the 5V rail (4.7 uF + 4 x 100 nF). Replacing them with a single 100 uF surface mounted tantalum cap reduced the noise by 30 dB. Replacing the tantalum cap with a leaded 4.7 uF electrolytic cap reduced the noise by a further 10+ dB (it dropped below the noise floor at the settings I was using). The noise was simply proportional to the ripple on the 5V rail. Using a long USB cable increased the ripple (to 100 mV pp) and thereby increased the noise. Many thanks to everyone who took the time to think about my problem and suggest solutions. And full score to David Hess and magic who suggested that the MLCCs could be to blame!

However, I don't have room for a leaded component. There's plenty of horizontal space, but only 1.5 mm free height for components. So now I'm looking for advice on quiet capacitors that would fit. Since that doesn't match the title of this thread, I'll start a new one: https://www.eevblog.com/forum/chat/quiet-capacitors/.

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