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Quiet capacitors
sveinb:
Hello good people, I am looking for advice on capacitors that don't vibrate. I'm making a USB microphone and have problems with an 8 kHz noise on the audio. It turns out that this comes from ceramic capacitors on the 5V USB supply rail. When you use long USB cables with active repeaters, you can get 100 mVpp ripple at 8 kHz because this is the packet rate on USB 2, and the microphone as well as the repeaters draw more current when they transmit. This causes the MLCCs to vibrate, which is picked up by the microphones. Replacing the MLCC capacitors with a surface mounted tantalum cap reduces the noise by 30 dB. Replacing the tantalum cap with a leaded electrolytic cap reduces the noise by another 10+ dB. However, I don't have space for a leaded component. There is plenty of horizontal space but only 1.5 mm vertical space, so now I'm looking for an ultra-quiet capacitor which would fit in that space.
SiliconWizard:
I don't know if such a thing really exists. One common approach to reduce the noise made by ceramic capacitors (when it's a definite no-go in a given design) is to decouple them mechanically from the PCB as much as possible - as it's the PCB that transmits most of the noise. For that, you can add two small slots on each side of the capacitor in the PCB. Alternatively you can try adding some conformal coating over the PCB to dampen the noise, but that will add production cost and is often not as effective as slotting.
selcuk:
Tantalum capacitors have less piezoelectric effect than many MLCCs so you may expect an improvement. The electrolytic capacitor may have a high ESR value. If tantalum has lower ESR than electrolytic, this ESR may dampen the oscillations. Can you try adding a series resistance to the tantalum capacitor?
magic:
C0G/NP0 dielectric MLCCs are supposed to be not piezoelectric, maybe an option if you need small values for local decoupling.
Not sure if it's a matter of oscillations and ESR will help, but large capacitance and a fairly high value series resistor (if you can tolerate the voltage drop) will act as RC filter to reduce voltage ripple at the capacitors (and the whole circuit). You will need to calculate if this is feasible in available space, budget and voltage drop limits. I hope you know relevant maths.
Zero999:
What value are the capacitors and how are they used?
Presumably they're in the signal path, rather than just supply decoupling.
C0G/NP0 shouldn't be piezoelectric, as mentioned above, but take up more room an are more expensive, especially larger values. Plain old aluminium electrolytic is also not piezoelectric and is much cheaper for larger values, but it's polarised and the tolerance is poor.
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