Which small ceramic capacitors have the worst microphonics?

Does anyone know which tiny MLCC have the worst microphonics? Brand, series?

I want to do some tests to see how well these things can sense low pitch sound, mainly via changes in capacitance but also piezo effects. Main reason is that I need a fairly low impedance, high capacitance and a very small size. Ideally less than 0.010" width and height but for right now I can test with larger ones. 0.050" or so would be fine for testing the concept, maybe even a little larger. A capacitance higher than

0.01uF would be nice. The frequency response doesn't have to go beyond 100Hz.
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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
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Wouldn't that most likely be X7R? ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

My suggestion, Joerg, is to get the greatest capacitance in the smallest size to start with.

Reply to
John S

I should have given some reasoning behind this. Sorry.

The high capacitance in small size can only be achieved, AFIK, with high dielectric constants. That means a lot of variation with applied voltage and with mechanical variation as well. They are transducers that were not designed as such.

Cheers

Reply to
John S

Well, the worst microphonics will be for quartz crystals and ceramic resonator equivalents (because they are poled). Microphonic response at frequency F depends on the crystal being asymmetric (so that it knows up from down). A non-polarized MLCC would see sound input at F and give feeble electrical output at 2F. Raise the bias voltage on the MLCC , maybe beyond the recommended applied voltage, to see electrical output at F.

Quartz crystals and ceramic resonators are usually packaged so you cannot stress them with external soundwaves, but the internal elements are sensitive. Ceramic resonators are permanently 'poled' with no need of a bias voltage source, and alpha-quartz has asymmetric crystal structure which amorphous polycrystalline quartz lacks. Sadly, you can't make a halogen lamp envelope into a good microphone.

Reply to
whit3rd

Yes, that's what I probably do. But it is often surprising, one brand has 2x the microphonics and all else is pretty much the same. So I was wondering if people knew something, such as "brand XYZ always has tons of microphonics" or something like that.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

recommended

source,

polycrystalline

I can't have any bias voltage in this case. But to my surprise I just read a report where a C0G cap is as "bad" in microphonics as the ferroelectric X7R sibling. This turf seems to be full of surprises.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

One would think so because it's ferroelectric but I found a Kemet study showing it may not be so in all cases.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

series?

Perhaps the brands from the unknown (read Chinese) manufactors may be worse since they might not have inhouse control to minimize that parameter.

Is this for a one-off or a production item. I wouldn't sleep nights if it were for a production item.

Regards

Klaus

Reply to
Klaus Kragelund

Long term it is for production but right now only a feasibility study. If it works we can then determine a pathway to obtain caps of consistent behavior. If necessary a custom run. Some deviation is fine because every unit will be calibrated.

I always sleep well :-) ... Drives others crazy, I can say good night, turn around, and 15 seconds later ... zzzzzzz

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

I'm the same way, even during periods of high stress on the job.

But, I have been told that could be a signal that one has sleep apnea, and it could be dangerous.

Some specialist say that if you can fall asleep fast, that is because you get to little sleep and you should get more sleep. I get along fine with 6 hours per night, but I may just be kidding myself....

Regards

Klaus

Reply to
klaus.kragelund

I thought all Vikings could do that :-)

Yep, especially if you have cases in the immediate family. The only way to find out would probably be in one of those sleep diagnosis centers that some hospitals run. Essentially you have to stay overnight.

6h is quite low. I try to get in 7h which isn't so easy anymore. Since California had a business exodus most of my clients are now 2-3 timezones ahead. This means I now have to get up latest by 6:30am. And then the dogs want to be walked as well.
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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

hasn't been my expereince. I once has some large like .5X x 1" stacked X7Rs from AVX in the lab as motor bus capacitors. When I ran the motor I could hear the RPM of the motor from the cap back across the lab. :)

Replaced them with Kemet, Novacap and another custom cap supplier, and they were very similar.

Can you gain some sensitivity by tuning the mounting structure (pcb I assume) to have a high mechanical Q at the frequency if interest?

Reply to
mook johnson

I've had differences and those most likely had to do with the number of layers and things like that. Some caps were taller than others.

Unfortunately not in this case because I have to measure from DC for

30-40Hz.
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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Much like chip design, you will probably find different parts of the PCB have more flex, which in turn effect the cap. Usually the corners are considered to be the quietest.

If you can't put the critical cap in a corner and if the PCB is rectangular. there is probably more flex in the long direction of the board, so place the caps perpendicular to the long edge.

There may be a way to parallel caps so the microphonics cancel. Obviously that would take more caps of a smaller value with different orientations.

If you dig into chip design books and papers, you run into this kind of black magic.

Perhaps flexible (elastomer) mounting of the PCB?

Reply to
miso

I actually want microphonics in this case and the cap won't be on a PCB. Some tests yesterday with what I have here showed that while slight pushing onto the surface of the cap does generate a detectable signal I could hardly measure any capacitance change. Capacitance change would be the only way to get DC out of it.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

On 14-04-2013 21:54, Joerg wrote: [snip]

All of these ceramic materials *are* ferroelectric, otherwise you wouldn't get the high epsilon_r. Slight changes in the composition of the compound affects temp coefficients and other properties, hence the classification into X7R, C0G etc., but they are all ferroelectrics.

I think it is impossible to deduce microphonics from these general material properties at all. For example I would assume that manufactures use tricks like varying the orientation of the layers in a multilayer cap to minimize the overall microphonics effect.

So most likely you have to contact the manufacturers and try to get into contact with one of the component design engineers knowing the gory details - which most likely is easier said than done :-(

Another loose ideas could be the IEEE Trans. Ultrasonics, Ferroelectrics and Frequency Control. I seem to remember an article on capacitor microphonics there, but a quick search didn't find anything. Most likely my memory doesn't serve me well here.

Or look for PVDF (Polyvinylidene flueride) sensors, usually used as piezo, but the should make a good microphone, too. Perhaps there is something on the market which can be mounted on a PCB. The price tag might be much higher than a capacitor, though.

Regards, Klaus

Reply to
Klaus Bahner

My experience with that is that they won't release much, this is usually closely guarded internal reseach.

Good idea. I am a member of that society but since the Transactions no longer come on paper it's easier to miss stuff when you don't make regular checking a habit. A paper kind of pushes itself under the nose for the evenings but the computers are off in our house after 6:30pm or so.

In this case it can be several Dollars per capacitor. But PVDF would not offer very large capacitances when small (a few hundred um by 1mm or so in real estate). I remember some PVDF experiments we did in medical ultrasound. We never warmed up to that material. It had great bandwidth which is important in that market but shows quite poor acoustic efficiency. So we kept PZT.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

You can easily test samples yourself.

A couple of years ago I submitted a TL431 based electret mic booster to Elektor magazine, which was published.

Among my experiments; I stripped a spare electret mic to get at the special JFET inside, I examined the microphonic properties of various components including bog standard capacitors of various types.

In case you have difficulty locating a copy of the article:

Give the TL431 a Vcc of 9 - 12V and a cathode load of at least 200 Ohms, connect the electret from cont ip to GND and set the K to 1/2 Vcc with a 47k from K to cont ip.

Measure the resistance - the final resistor gives large nfb, both AC & DC, you need to shunt the AC nfb to get very high gain with stable DC set point. connect a 100uF cap to GND and a 1k2 from K to the + of the cap, subtract that 1k2 from the measured resistance and put a fixed resistor equal to the remainder from cap + to cont ip.

If yoy break open the electret mic capsule, you can remove the electret dielectric and attach wires to connect your capacitor.

Pretty much all types of non-electrolytic capacitor are microphonic when tapped with a screwdriver - you will soon see, some more than others.

If the finished circuit picks up radio stations, your electrolytic has low ESR - put a 100 Ohm resistor in series with it.

Reply to
Ian Field

You don't disclose the application, and the devil is always in the details.

I have a couple of concerns.

A device that small and stiff is gonna have about zero sensitivity below 100Hz. To make it work, you'll have to "bend it" by attaching it to something bigger and flexible. Operational bending is bad enough. Shock/vibration/resonance is gonna be a reliability nightmare.

I'm skeptical about component usage far outside the design parameters. I've had enough production problems when the vendor actually improved a published spec. Exploiting an unspecified characteristic that's undesirable for the typical application is asking for trouble.

Reply to
mike

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