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| Buck converter that draws a constant current |
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| NiHaoMike:
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. |
| sveinb:
--- Quote ---Quote from: sveinb on January 25, 2024, 11:32:34 am --- Quote ---I've identified two routes that this noise takes: 1. The 5V supply voltage gets an 8 kHz ripple which leaks to the microphone supply voltage and 2. The current variation on the USB voltage rail causes mechanical vibration of the circuit board which is in turn picked up by the microphones. I cannot move the components to different circuit boards. --- End quote --- How exactly do you know it, particularly the latter, or is it only guesswork? Is this a digital mic or analog, feeding an ADC in the USB chip? If the latter, I suppose you may need to consider the ADC's PSRR too. As a quick and dirty experiment, I would try powering the mic with external batteries and see if it helps. --- End quote --- 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. 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) |
| sveinb:
--- 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. |
| sveinb:
Someone outside of this chat pointed out another thing I hadn't considered: As mentioned, the noise problem gets more pronounced when using long usb cables with active repeaters. The longer cable means that the 5V supply gets softer and more susceptible to ripple caused by the USB interface's varying current draw. However, the 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. |
| tom66:
Fundamentally you will usually have no control over the USB source. You can probably assume that the computer could put 100mV of ripple and noise on the USB supply at 8kHz and still be compliant with the USB specification. As far as I can tell, USB makes NO requirements about the noise on VBUS, it only specifies a minimum voltage under load and things like that. So you will need to build your product to withstand supply noise and not try to stop it happening in the first place because someone will just invent a noisier source even if you do manage to keep your current nice and close to DC. |
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