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A noise of ceramic capacitors?

Discussion in 'Electronic Design' started by Vladimir Vassilevsky, Sep 8, 2007.

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  1. Hello All,

    There is a piece of schematic which contains the 0.22uF X7R (0805) in series
    with 200k. The cap is biased to 2.5V. It looks like the capacitor (!) is
    producing a lot of low frequency noise in the area of several Hz. And this
    noise doesn't seem to be due to the microphonic effect. The replacement of
    the capacitor to a film or tantalum type helps.

    Do you know why there could be a low frequency noise in a ceramic capacitor?
    Am I missing something?

    VLV
     
  2. qrk

    qrk Guest

    Leaky or cracked capacitor? With a 200k impedance, leaky caps will
    cause issues. Could you have a contamination problem on the pcb? I
    have come across ceramic caps and pcbs with low megohm leakage
    problems. Which reminds me, I gotta install the leaky cap in my Tek
    465 scope.

    Mark
     
  3. Maybe air currents causing temperature variations, which vary the
    capacitance. If you have a capacitance variation, and a bias voltage,
    you have voltage variation, since the charge remains constant. A C0G
    type should be much better, but not cheap or small.


    Best regards,
    Spehro Pefhany
     
  4. I know you already said - but are you absolutely sure it is not
    microphonic? I have just had to replace some X7R's with tantalums, in
    critical areas of a circuit. They were definitely badly microphonic -
    e.g. mV-level pulses when the circuit board was tapped. I think this
    extends to down DC, i.e. a strain in the body of the capacitor would
    produce a level shift (in absence of any other currents).

    Other than that - X7Rs have quite a large temperature coefficient the
    capacitance changes so will the voltage, at frequencies above 1/2piRC.
    If there are fast temperature fluctuations I guess this could be a
    problem too.
     
  5. Tom Bruhns

    Tom Bruhns Guest

    For what it's worth...about 15 years ago, I was working on the design
    of a circuit where I used a ceramic cap, 1206 SMT 0.1uF X7R, for AC
    coupling, but in a different arrangement than usual. I discovered a 1/
    f noise that was not apparent until you got below 10Hz or so, due to
    the capacitor. It would not have been microphonics, and unlikey due
    to temperature variations, as the spectrum was too cleanly 1/f. (It
    takes quite a while to get statistically-significant spectra at such
    low frequencies...) Polyester film cured the problem. It only
    happened with a bias across the cap, but the amplitude of the noise
    was nearly independent of bias voltage above a low level, around a
    volt as I recall.

    Cheers,
    Tom
     
  6. Joerg

    Joerg Guest

    Hmm, I did a laser diode controller earlier this year where caps were in
    a PID loop. Provided extra pads for film caps just in case. Found no
    difference between ceramics and film. And this thing is ultra-sensitive
    down to the uA level, resp, uV level at the caps. Noise in the sub-10Hz
    range would have killed things but it didn't happen. Could there be a
    bad lot or brand of caps involved here?
     
  7. Mook Johnson

    Mook Johnson Guest


    What was the p-p amplitude of the noise? Are you talking about 10s of mV or
    uV level?

    If you're talking about 10S of mV level then I'd scrutinize the measurement
    method. Is the voltage being measured directly across the suspect
    capacitor? The fact that the film or tantalum cap only helped but didn't
    competly cure it seems to eliminate the micrpphonics as a possible cause.
    Possible low frequency coupling into the high impedance node?

    If your in the uV range there a re a variety of issues that could cause
    that. rapid temperature swings of a few degrees C. Low frequency coupling
    (electro or magnetic) into the high impedance node. Ceramic capacitors have
    been found by some of the analog gurus at my company to have relatively high
    levels of "popcorn" noise when used in high gain amplifiers. Glass or
    teflon solved the problems.
     
  8. PeteS

    PeteS Guest

    Ceramic caps vary their capacitance with applied voltage which may cause
    you some issues and might show up as phase noise, which would be more
    apparent with low frequency, large signal swings. Much depends on the
    rated voltage of the device. A typical X7R will have 50% rated
    capacitance at rated max voltage.

    Cheers

    PeteS
     
  9. What bias voltage did you have on the caps? The OP has a huge bias
    voltage-- 2,500,000 uV, so a mere 10ppm change in capacitance means
    25uV of variation. That's less than a mK at the maximum tempco of
    0.15%/K.


    Best regards,
    Spehro Pefhany
     
  10. Tom Bruhns

    Tom Bruhns Guest

    Nope, does it with different lots, different brands, all at about the
    same level. But you would never notice it in most circumstances. I
    can't imagine you'd see it in a controller.

    Cheers,
    Tom
     
  11. Fred Bloggs

    Fred Bloggs Guest

    You basically have metal on ceramic junctions in that thing, it must be
    a low frequency distribution on the leakage current. Switching to metal
    on film eliminates the troublesome physics altogether.
     
  12. Joerg

    Joerg Guest

    Roughly a volt, so in the same ballpark. Temperature changes are really
    slow. Got to make sure this isn't in any area of turbulent airflow. But
    ok, since mine was in a PID loop slow changes are muffled. However, the
    host can hold this board in software lock where the PID is disabled and
    that's much slower. No problems there either. We noise-tested the heck
    out of this design.
     
  13. Joerg

    Joerg Guest

    We would at least see it while it's in software lock or hold mode. With
    a laser diode that is wavelength-steered and in this application a few
    microvolt change on that cap would have thrown us out of the range where
    the whole thing worked at all. IOW a red light would have come on.
     
  14. Fred Bloggs

    Fred Bloggs Guest

    If the noise was inside a loop correcting down to DC and there was a lot
    of gain between the integrator and the error detector, it would have
    been suppressed by the feedback.
     
  15. Joerg

    Joerg Guest

    Yes, but as I said I had designed in a feature that allowed the SW to
    stop the loop and SW-control the whole thing. Which my client ended up
    using more extensively than we all thought, they kind of got a free
    production tester out of this whole design. No noise problems there either.
     
  16. Benj

    Benj Guest

    None of this is unusual. Ceramic caps are cheap and have relatively
    high capacitance for the size but the ceramic dielectric is NOT the
    most stable stuff in the universe. They are prone to lots of noise
    especially when there is considerable leakage. The dielectric in the
    higher value ones are worst. If you want a pure capacitance rather
    than a capacitance in parallel with a noisy leakage resistance use a
    capacitor with a decent and stable dielectric. These include mylar and
    other films, mica, glass, or any other stable dielectric. Note that
    this does NOT include tantalum. Though apparently your tantalum was
    better than the ceramic ones. The trade-off of course is that
    capacitors with better dielectrics than ceramic or tantalum are much
    larger and often more expensive too. Sometimes MUCH more expensive.
    If you want more details about this sort of thing, talk to some of the
    analog guys who design high end mic pre-amps and the like.
     
  17. Tom Bruhns

    Tom Bruhns Guest

    Microvolts are large compared to what I saw. The circuit in which I
    saw the effect put the capacitor in an unusual electrical environment
    in which the effect was observable; it would not be in any common
    circuit configuration I can think of. I regret that I'm not at
    liberty to tell you more about the particular configuration. It's
    possible that Spehro's comment about thermal variation would explain
    it, but as I recall there were reasons I thought that was not the
    answer (like, the amplitude didn't vary nearly enough with changes in
    DC bias).

    Cheers,
    Tom
     
  18. Tom Bruhns

    Tom Bruhns Guest

    WHAT tantalum one?? -- Leakage current is an interesting suspect for
    the noise I saw in the X7R ceramics. If I get a chance, I might check
    that out. On the other hand, lumping all ceramics together as "bad"
    is not very useful. C0G caps are available in reasonably large values
    these days, and are better in many ways than Mylar/polyester film
    caps. Maybe I can start some leakage tests on various ceramic
    dielectric caps, to go along with the ones I have running on metalized
    polyester and polypropylene ones. Both of those are showing longer
    self-discharge time constants than you would suspect from the
    manufacturers' ratings.

    Cheers,
    Tom
     
  19. Joerg

    Joerg Guest


    Microvolts are large compared to what I saw. The circuit in which I
    saw the effect put the capacitor in an unusual electrical environment
    in which the effect was observable; it would not be in any common
    circuit configuration I can think of. I regret that I'm not at
    liberty to tell you more about the particular configuration. It's
    possible that Spehro's comment about thermal variation would explain
    it, but as I recall there were reasons I thought that was not the
    answer (like, the amplitude didn't vary nearly enough with changes in
    DC bias).
    [/QUOTE]


    Awww, now you held a delicious piece of crispy bacon under all our noses
    and then pulled it away ;-)
     
  20. Joerg wrote:



    Joerg,

    Didn't the shaman tell you that gluttony is one of the sins?

    VLV
     
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