C0G/NP0 microphonic/piezoelectric vs Film

[b_force]

It's well know that electrically C0G caps perform very good (distortion and noise)
Even better or in the same league as good quality film caps.

Other ceramic caps are well know to be pretty microphonic.
Besides the fact that it can be pretty annoying in your multimeter (lol), it can really ruin your day if being used in an high frequency high current circuit.
(bizarre weird coupling effects, feedback loops, you name it)

Unfortunately I can't find any decent data and measurements on piezoelectric effect of C0G vs film capacitors.
The internet is full of measurements about distortion and all, but that's where it stops.

Because the distortion (and noise) levels are pretty good, my feeling says that the piezoelectric effect is also very minimal (like film)
Otherwise you would see it back in the noise and distortion.
I only would like to see some good measurements to back it up.

I am families with a who bunch of articles, like: "Advances in Class-I C0G MLCC and SMD Film Capacitors" by Kemet.
But again, non of these go really into detail about the piezoelectric effects (just say it's minimal or very low).

Anyone an idea, references or measurements?

[tszaboo]

Not all COG caps are created equal. It will be different for different values, different voltages, different manufacturers. I HAD to replace X5R or X7R capacitor with film, because it was too microphonic. You can test them, it is really simple. Just solder an capacitor to an RC filter, (for me, R was some KOhm, C was some 4-10uF) connect some stable voltage to it, like 5V, drop a small object, like a pen on top of it, like from 2-3 cm. Measure the output on a scope.
I'm sure someone could make a nice video from this phenomenon (wink wink).

[T3sl4co1l]

C0G does not exhibit piezoelectricity.

They probably exhibit a small amount of electrostriction, which informally might look the same (an electromechanical coupling), but is a different phenomenon.

Most materials exhibit this property, as well as the analogous effect, magnetostriction.

If nothing else, you can think of the electric attraction between the plates acting to compress the dielectric, which has finite rigidity (elastic modulus), and therefore increasing the capacitance slightly at the same time.  (That change may manifest as a dependent increase in value, or as a loss component.  That change is likely swamped by much stronger effects, such as saturation in ferromagnetic cores.)

As for the magnitude, if you can figure out the composition of the capacitors, and find academic articles discussing the material properties, that's probably the best you'll find.

In complete capacitors, it seems like an ill-posed problem: there is no obvious preferred direction to apply force to a chip capacitor, that would be both general and useful.  So I wouldn't think you'd find an article discussing that particular application, or if you do, it may not be very useful.

(Yes, applying pressure in the "stack of plates" direction would be the most obvious -- but a board-flex strain is most likely to be applied in a real circuit, which is very different.  Conversely, it's not obvious how one could quantify board-flex strains: it depends on solder amount, pad size, chip height, board material and temperature, and so on.)

For the same reason, it might be more beneficial to measure things yourself.  Don't forget to measure with an applied bias (polarization): if the material is electrostrictive, you will observe an output proportional to bias.  Ideally you'd have an accelerometer on the board as well, but if you simply whack the board in a consistent manner (hammer it with a solenoid, in a repeatable setup?), that would be okay for qualitative comparisons between component types.

Tim

[b_force]

Thinking a bit more; C0G/N0P are not ferroelectric.
Does that automatically mean there is absolutely no microphonic effect?
I mean is 'not ferroelectric' meant in an absolute sense, or in a relative sense (as in, so low, it's as good as nothing)

Or does it indeed depends on what NANGBlog says how the capacitor is being made?

[Kleinstein]

X7R and similar high capacitance materials are a totally different class, with ferroelectric material.

C0G caps are not ferroelectric (at least if the material is pure). So ideally no piezo-electricity but only the much weaker electrostriction.
Also with CoG material not all caps are made from the same material - smaller form factors will use higher K material and thus a possibly higher interaction. Usually the effect is considered very small and is not specified.

I am not sure film caps are better in this respect - in a few cases one can hear them.

One difference is that the ceramic material is stiff and likely low in mechanical loss. So there will be rather sharp resonances (e.g. 100s of kHz range) in the response, where the effect will be higher. Also the shape and coating of the caps can make a big difference. THT caps with a plastic coating might be good candidates to get low coupling.

[tszaboo]

Thinking a bit more; C0G/N0P are not ferroelectric.
Does that automatically mean there is absolutely no microphonic effect?
I mean is 'not ferroelectric' meant in an absolute sense, or in a relative sense (as in, so low, it's as good as nothing)

Or does it indeed depends on what NANGBlog says how the capacitor is being made?

The shape still changes, if you apply pressure/shock/board flex. Imagine, they change the distance of the plates, so capacitance changes, and the charge will change the voltage. The same amount of electrons, in a different capacitor...
I only know about the microphonic effect, that is why I was talking about that.
Really, it will only matter, if you know, how much effect can you tolerate. And since whe already have to beg for the very important C/U curves from capacitor manufacturers, I would be very surprised if there would be any quantitative data for the microphonics.

[b_force]

For the same reason I miss the permissible AC voltage (Vrms) vs frequency graphs a lot.
Because a 63Vdc (40Vac) capacitor, can't handle much more at 10-20kHz (or more) anymore.
Sometimes another reason to put multiple in parallel or series.
Through hole are worse in this.

[DaJMasta]

I don't understand all the physics that goes into it, but I did research a bit to try to find this information for a precision audio design, and I found the same result, very little information is actually available on specific parts or series.  I got the impression that there are so many small changes on the physical level that it was simply too difficult to get everything into a datasheet and update it every time the manufacturing process changed just a little.

It's my understanding that any variations in the dielectric would change its response, but the physical area of each stacked layer would change it (larger is worse) and the number of layers used would effect microphonic response the same way.  That would mean a single manufacturer's single series of caps would effectively need a different response graph for every value produced, and that would likely change with the purity of the dielectric and if they came from different factories/etc.


The conclusion I came to was that if you wanted to be sure.... you really just had to buy the parts and try to characterize them to see whether they were going to be a problem.  Physically smaller caps with lower values should pick up less and won't pick up low frequencies as well, but whether or not that would be a problem... it can be really difficult to say.  Combining several values as you would do with bypassing may be the best way to avoid microphonic effect pitfalls, aside from just insulating your board from sound impulses.  I've been under the impression that film caps are still better than C0Gs, but I think if nothing else, axial caps with a spiral wound construction should probably be better than other dimensions because it minimizes that cross sectional surface area to pick up sound, and inner windings would be somewhat more sound insulated.

Again, I haven't done the characterization and doubt I have the knowledge of physics to be any kind of authority on it, but that's my understanding after looking into it a bit.

[nctnico]

An important difference between ceramic and film is the leakage current. If you are building circuits where a few nanoamps of leakage can cause trouble then film capacitors are the better choice.