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.

[magic]

Yep, that's what happens when OP decides that the topic has changed enough to start a new thread and doesn't link to the old one.

(They are 100% decoupling, all analog bits are fully integrated).

[Berni]

All of the high dielectric constant MLCC capacitors will have a pronounced piezzo effect.

You can get MLCCs that come with built in J lead style standoffs on the pins to lift the capacitor off the board a bit, those might reduce the problem some but not eliminate it, they are also fairly exotic parts.

As said above NP0/C0G type MLCCs will have a much less pronounced piezzo effect, but they are only available in small capacitance values.

Electrolitics are typically too big even in SMD packages indeed, but you can get tantalum capacitors in pretty decent capacitance values from the small SMD parts. Tho the thing to watch out there is that they are sensitive to overload (they literally explode and catch fire), so careful with putting one of those directly across the USB input.

[magic]

Another thing about USB ports is that the spec limits connection surge current, so that devices already on the bus don't brownout.
IIRC one device is only permitted up to 10μF without series resistance.

[SeanB]

Simple enough with tantalum to put a series resistor, either 1R for small ones, or just a ferrite bead for larger current, as they will work well enough as filter and current limiting, plus absorb some of the energy in ringing.

[Zero999]

Yep, that's what happens when OP decides that the topic has changed enough to start a new thread and doesn't link to the old one.

(They are 100% decoupling, all analog bits are fully integrated).

Then it's also possible the low ESR capacitors are causing a low drop-out regulator to oscillate, in which case a series resistor/ferrite bead will help to reduce the Q.

Another option is to add an RC filter to the power rail of each microphone. Obviously the capacitor needs to be electrolytic/tantalum, which is non-microphonic.

[sveinb]

I tried putting a 5 ohm resistor in series with the usb power. That didn't really help. I guess the problem is that most of the ripple is created by my own circuit, and is not coming from the outside. (Edit: maybe some of you are suggesting that I put the resistor between the capacitor and the voltage regulators, but I'm nervous about the stability of those regulators in that case)

[jonpaul]

check for SMPS unstsable/sub cycle oscillation

j

[EPAIII]

Just send me your oversized ones and I will run them through my hydraulic press. I guarantee they will fit.

[Zero999]

(Edit: maybe some of you are suggesting that I put the resistor between the capacitor and the voltage regulators, but I'm nervous about the stability of those regulators in that case)

It's possible the ESR of the capacitors is too low, which is resulting in instability. Look up low-dropout regulator capacitor ESR stability, using a search engine. It's a very common and well-known problem, which needs to be ruled out.

[magic]

I tried putting a 5 ohm resistor in series with the usb power. That didn't really help. I guess the problem is that most of the ripple is created by my own circuit, and is not coming from the outside.

Yes, sorry for suggesting this, it was nonsense
It would only help if the ripple were coming from other devices on the bus.

It is also unlikely that an oscillating regulator produces 8kHz, they usually oscillate faster. Your USB microframe theory is more plausible.
Besides, it's not that hard to find LDOs stable with any reasonable ESR.

If there are regulators on board, their output capacitors shouldn't see much ripple and produce much noise, so the problem is the input capacitor.

Since 100μF tant was still not good enough, maybe try a 10μF one rated for higher voltage. It may or may not be quieter. Lower capacity will not violate the USB spec. Increased voltage rating may increase its reliability. Pay particular attention to a transient voltage peak which occurs after the device is connected and all capacitors have just charged up but cable inductance keeps pushing more current into them. This can destroy underrated voltage regulators or capacitors.

[T3sl4co1l]

Tantalum and aluminum polymer are fine candidates here.  I would avoid Ta for price and conflict reasons.

C0G aren't available in large enough values for bypass, and film would be way too big, but they are considerations at higher voltages, where the cost is justifiable and the energy density is good.

Tim

[sveinb]

I'll try with some smaller tantalum caps and route a slot around them to hopefully isolate them a little bit from the rest of the pcb. The smallest aluminium polymer caps I could find were a little bit too big.