short-term stability of class-1 ceramic capacitors

It appears that the capacitance of class-1 ceramic capacitors (NP0=C0G,

N750, etc.) in general suffers changes of the order 10^-4 over short periods of time. See the p.68 of the Vishay-Draloric General Information on Ceramic Single-Layer Capacitors under

Apparently, the inconstancy can somehow be reduced to below 10^-6. Is that done by special processing or treatment or by selection? Does anybody know the physical mechanism behind the inconstancy? And what is the typical time scale of the "short-term" fluctuations?

TIA, Martin.

Reply to
clicliclic
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Hello Martin,

Maybe by baking well above 100C. Just keep in mind that a stabilizing process can be partly spoiled by the heat during the solder process.

Regards, Joerg

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Reply to
Joerg

If the dielectric's that 'touchy' and that's problematic for you then why not use another one ?

You're obviously looking at small values so mica might be good for you.

Graham

Reply to
Pooh Bear

You seem to be thinking in terms of internal stresses in the ceramics. Stress relief is in fact behind the aging of class-2 ceramics, the constituents of which are ferroelectric (and hence piezoelectric) - class-2 caps show a logarithmic capacitance reduction with time. The oxide mixtures of class-1 dielectrics don't contain ferroelectric components, although the different crystallites will no doubt be stressed when the fired mixture cools during production.

I will try accelerated aging or stress relief treatments as a last resort - if selection within one lot, replacement with caps from different lots and/or different vendors doesn't help (10^-6 caps are ok

for my purpose; the "noise" level of some 100V 2% EGPU caps made by Philips/BC Components with N750=U2J dielectric is obviously worse than 10^-5).

Martin.

Reply to
clicliclic

G,

Reason is: class-1 ceramic caps can be had with virtually any tempco from

+100 ppm/=B0C to minus-a-few-thousand ppm/=B0C. Mica is always around +40ppm/=B0C, I think.

I am surprised by the magnitude of the inconstancy: 10^-4 is not automatically insignificant, these caps are used all the time, and no other manufacturer is even mentioning the problem!

Nor have I been able to locate information on this topic on the wider internet.

Martin.

Reply to
clicliclic

Vishay states: "For higher requirements e.g. in commercial applications, we can supply *tubular* capacitors, the inconsistancy of which has been reduced to a minimum."

- This makes me guess that the inconsistancy has something to do with a mismatch in TCE (thermal coefficient of expansion) of the SMD package with respect to the PCB material the capacitor is mounted on. The stress and creep typically cause sudden stepwise changes. Using better quality PCB material (polyimide, LCP, Rogers) can also help reduce this stress. Another thing: don't forget that the PCB material properties determine the stray paracitic capacitance. These may be in the same order as the 'inconsistancy'.

According to Vishay their high grade parts, OR their 'tubular' parts also solve the problem.

Reply to
KoKlust

I downloaded the pdf file and I have to admit that this was the very first time I ever heard of this phenomenon. It may explain why HP used some small tubular caps in the 10MHz clock oscillators in the frequency counters I commonly work on.

My guess would be that the thin wall tube allows for a more uniform finished crystal structure in the ceramic.

-

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

-----------------------------------------------

Reply to
Jim Adney

A footnote: "inconsistancy" must be a typo for either "inconstancy" or "inconsistency". The former is much to be preferred in the present context.

I agree that mounting-related stress has to be considered for SMD parts (I am not sure Vishay make single-layer SMD parts). However, Vishay's introductory statement on p.68 is not restricted to particular capacitor styles, and so refers to ceramic single-layer caps in general, making sense primarily for class-1 dielectrics as the ferroelectric class-2 types aren't very stable anyway (their capacitance depends on AC voltage and DC bias, on temperature, and on capacitor age).

The introductory statement reads: "During operating a ceramic capacitor the capacitance value may change for short periods of time." In fact, I don't see why this shouldn't apply to multilayer caps as well, regardless of their mounting (leaded or SMD).

I've thought about PCB contributions: they easily exceed the order of the inconstancy, but their short-term variation should not.

Rather, as far as I could see, their (electron-tube aera style) tubular RDQL (lacquered) and RDQT (resin coated) caps are the *only* models for which controlled "short-term stability" (or "KzK") grades are offered (with KzK from 4 to 6). All others models are "uncontrolled" - they aren't graded. See the RDQL/RDQT datasheet.

Martin.

Reply to
clicliclic

Hmm okay! So your problem is solved?

I am very curious if the Vishay guys know the science behind this phenomenon. Did you ask them already?

Reply to
KoKlust

Indeed I contacted them using the e-mail address given at the bottom of the single-layer-capacitor pdf files. I asked them to furnish (or point me to) more detailed information, in particular as to the time (or frequency) scale of the fluctuations. I got an automatic forwarding message right away on Monday morning, but nothing since (and I'm not holding my breath).

True, my problem would be solved if I had RDQL/RDQT caps with KzK=6. I don't like their footprint though, and haven't looked into availability of the different tempcos. Instead, I am thinking of checking various class-1 caps using a simple setup (what Vishay would call a "very sensitive measuring device") like this:

The capacitor under test is connected via a large resistor (say 22Mohm) to a film-capacitor stabilized DC voltage of around 10V, the capacitor voltage amplified by something like a TL071 configured for a gain of

100 or 1000, and the noise near f = 1/(2*pi*R*C) observed on a scope. This should allow to probe dC/C = 10^-6 for 100pF caps, the limiting factor being white noise from the 22Mohm (600nV/rtHz, smaller resistors would mean that one sees higher frequency components only), and the input current noise of the TL071 (about 10fA/rtHz at 1kHz) multiplied by 22Mohm.

Perhaps interested people could share their results on the group?

Martin.

Reply to
clicliclic

Wow, this is a completely new way to measure capacitance for me! Did you ever use such a setup successfully? Up to ppm scale? I'm afraid I cannot follow how you arrive at a ppm capacitance measurement only with a RC, amplifier and scope.

Didn't you forget to mention something like a demodulator / 72 Hz bandpass filter / rectifier? Else I don't see how you can make picofarads from the noise on the scope screen. Reading the amplitude of the noise gives peak-to-peak values that get as large as you want, if you wait long enough. And even with a spectrum analyzer it would take a long time to get anywhere near 1 ppm.

Well anyway you probably have an easy explanation.

Best regards,

Marco

Reply to
KoKlust

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Well ... the idea is not to accurately measure capacitance, but to roughly grade ceramic caps according to their short-term stability - by converting capacitance fluctuations into voltage fluctuations: 1ppm *

10V =3D 10=B5V. Surely one can decide if there is 1mV or 100=B5V or 10=B5V of noise without waiting forever? But I haven't tried it yet, as I am currently working on something else.

As some people may have problems distinguishing noise components around

72Hz (for R =3D 22Mohm, C =3D 100pF) on a scope by eye, a bandpass would come in handy; 6dB damping per octave above and below the observation frequency can easily be implemented in the TL071 feedback path. Alternatively, one might directly observe the noise spectrum in the frequency domain, using a digital scope ("spectrum analyzer").

Note that the RC combination at the OpAmp input already provides for damping of both the resistor noise and the input current noise above

72Hz.

You never stop learning!

Martin.

Reply to
clicliclic
  • cut *

Well ... the idea is not to accurately measure capacitance, but to roughly grade ceramic caps according to their short-term stability - by converting capacitance fluctuations into voltage fluctuations: 1ppm *

10V = 10µV. Surely one can decide if there is 1mV or 100µV or 10µV of noise without waiting forever? But I haven't tried it yet, as I am currently working on something else.

OK, thanks for the explanation. Maybe for a rough classification, your method will work. But I'm afraid it runs into problems on the 1ppm stability measurements. 1ppm is really very very hard to measure.

Your setup can be improved a lot if you leave out the 22meg resistor, replace the 10V dc source by a triangular (or square) wave generator giving several volts of amplitude, and demodulate the voltage at the opamp output synchronously to the wave generator.

You will also want to use a lower noise (order

Reply to
KoKlust

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Actually, I'm proposing to do with class-1 ceramic capacitors what one usually does with a condenser microphone.

I hope to be able to try it next weekend and will let the group know the=20 result, positive or negative.

Martin.

Reply to
clicliclic

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I've now done the experiment for various class-1 ceramic capacitors (EGPU made by Philips/BC Components) around 100pF. They were charged to

5V DC via 10Mohm, and the signal was picked up by a TL071 configured for a gain of 1000, with 6dB per octave damping below 1/(2*pi*R*C) =3D 160Hz (the feedback elements were 1Mohm between OUT and IN- and 1kohm in series with 1=B5F between IN- and ground). Electrostatic shielding was required to prevent 50Hz pickup.

No obviously capacitor-generated noise contributions were found on any capacitor under test. The noise with ceramic capacitors was indistinguishable from that with film capacitors and agrees with the expected resistor noise of 400nV/rt-Hz (with 10nV/rt-Hz at 1kHz, the current-noise contribution from the Tl071 should be somewhat below this

level).

Thus, if class-1 ceramic capacitors exhibit capacitance fluctuations of the order 10^-4 to 10^-6, the frequency spectrum must concentrate below

160Hz. If the fluctuations proceed in sudden steps (i.e. with sub-millisecond rise/fall times) - as one might perhaps suspect - individual steps for the tested capacitors must be below a few times 10^-6.

In order to learn more, the experiment should be repeated with R=3D1Gohm and an AD820 (about 2pA bias current, about 18fA p-p from 0.1Hz to

10Hz) in order to probe frequencies down to 1.6Hz.

Martin.

Reply to
clicliclic

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