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Noise in tantalum SMT caps?

Discussion in 'Electronic Design' started by Spehro Pefhany, Jan 18, 2008.

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  1. Any idea what noise I'd see in a tantalum 150uF cap with 1.25V across
    it?


    Best regards,
    Spehro Pefhany
     
  2. Joerg

    Joerg Guest

    A very loud bang?
     
  3. Phil Allison

    Phil Allison Guest

    "Spehro Pefhany"
    ** This a trick question ??

    Like how many ohms in a coulomb ?



    ........ Phil
     
  4. Yes I am puzzled too, although he must have some reason for asking...

    I did find that tantalums have very low microphony (I could not detect
    it) compared to high K chip ceramics which have lots. It makes them a
    good choice for filtering any kind of reference voltage.
     
  5. Guest

    Celine Dion?
     
  6. Boris Mohar

    Boris Mohar Guest

    Polarity marking is backwards so it is starting to break down?
     
  7. John Larkin

    John Larkin Guest

    I'd guess just the thermal noise of the esr, which is only a couple
    tenths of an ohm maybe.

    John
     
  8. No sub-10Hz voltage noise as aluminum electrolytics under bias are
    supposed to have?

    I'm looking at a design (not my own) where there's an RC on the output
    of a voltage reference feeding a Sigma-Delta 24-bit ADC. The R is
    causing a full scale gain error of, oh, 17bits. 8-( Over 1%.

    Is the reference input current constant with input voltage?
    Temperature? I doubt it. 8-(
     
  9. MooseFET

    MooseFET Guest

    Look at the leakage current. The electrons rattle as they go
    through. The more leakage, the more noise.
     
  10. John Larkin

    John Larkin Guest

    Dunno. I recently tested some standard alums and some tants, posted
    elsewhere, and the alums had serious chemical things going on inside;
    the tants and polymer aluminums much, much less.
    It certainly varies as a function of adc activity, which can vary in
    some systems, even for delta-sigmas.


    Well, the cap-versus-temp bahavior of a tantalum isn't great. So if
    there were standing DC on it, as there is here, minute temperature
    fluctuations will modulate C and, since CV is conserved, V would
    wobble around. The same thing would happen on hi-K ceramics.

    The R is maybe a bad idea, if unbuffered by an opamp.

    John
     
  11. Fred Bloggs

    Fred Bloggs Guest

    There most certainly is and it has been characterized as 1/f^2 down to
    the millihertz region. From what I gather from the research papers
    available on the web, most of this noise is due to thermal instability
    around dielectric microleakage areas which self-heal via a chemical
    transformation of MnO2->Mn2O3. These are characterized by a gradual
    buildup of noise to a peak and then a substantial drop once the healing
    has occurred. Also the very construction of the things makes them a
    hotbed of tunneling and current trapping mechanisms as well as
    possessing a nearly shot noise effect due to a
    metal-insulator-semiconductor conduction mechanism from anode to
    cathode. And all of these things are not even close to stationary,
    varying all the time with a very slow trend into deterioration with
    aging. This is all transparent to the majority of applications requiring
    bulk capacitance at low ESR in small packages, but entirely visible as a
    reference for a 24-bit A/D conversion application.
     
  12. Joerg

    Joerg Guest

    Ouch! Did they split the grounds? That can cause part of it. I've never
    had a case where that brought the ENOB down this much, but it put a
    measurable crimp into it.

    There's probably a lot more that's not constant.
     
  13. John Larkin

    John Larkin Guest

    What's the magnitude of the currents? The source impedance of the
    reference (or of the series resistor) then determines the noise
    voltage. I've done analog systems that were good to a few ppm
    stability, with tantalums bypassing refs and excitation amps.

    The resistor is a bad idea. What's its value, Spehro?


    Numbers, gentlemen!


    John
     
  14. Tom Bruhns

    Tom Bruhns Guest

    I've used tantalums at a point in a noise-filtering circuit where I'd
    for sure have noticed noise much above 10 nanovolts/rtHz, and not seen
    any. If you'd like, I could make a measurement with an AD797
    amplifier connected to a couple biased-up caps, driving an HP89410
    analyzer, and get a noise spectral density plot. I'm curious enough
    about it now I may just do that anyway...

    Cheers,
    Tom
     
  15. Joerg

    Joerg Guest

    Same experience here. Tantals are either very quiet or very loud (as in
    *BANG*). Quite nicely suited for filters if the tolerance were smaller.
     
  16. FWIW I would love to see this. I have been using e.g. 2k2 feeding a
    22uF tant as a reference noise filter. (Feeding a high impedance opamp
    input). As I mentioned chip ceramics are a disaster here because of
    microphony and perhaps temperature fluctuations.
     
  17. Joerg

    Joerg Guest

    Can't say that. I use ceramics all the time, never had such issues in a
    Ref distribution and some of that is rather sensitive.
     
  18. IIRC I measured about a mV disturbance across the cap, when tapping
    the PCB. This was when using some 0805 1uF X7R. I had to go through
    the circuit and replace them with tantalums, in the critical areas,
    which fixed the problem.

    ref-[R]-.----- opamp> --.
    [C] LED ----> Photodiode -> AC amplfier -> output

    Basically I was looking for a small fluctuation on a large background
    signal.


    Hook up a 1M resistor and a 10u chip ceramic, with wires, to a voltage
    source. Put a scope on AC across the cap. If you move the wires you
    strain the body of the cap - this results in a visible output voltage
    change.

    I assume the same thing is going on when the PCB is flexed.
     
  19. Joerg

    Joerg Guest

    I'll try that. Interesting, I always give my circuit boards a good
    banging and never saw issues like that. Of course we don't have to use
    RoHS solder out here :)
     
  20. Tom Bruhns

    Tom Bruhns Guest

    Have you tried C0G ceramics? You can get 0.1uF in 1206 size these
    days, though a bit pricey.

    When I get a chance I'll set up the (rather simple) noise spectral
    density thing on the tants. Doubt I'll do any microphonics testing on
    them though; it'll just be to see if they are stable under bias. Per
    John Larkin's comments, I'll try it in "free air" and with some sort
    of thermal insulation to at least slow down the changes. That's one
    area where C0G should shine. Though they are rated +/-30ppm/C, you
    can get batches that are much lower. I accidentally mixed up some X5R
    and some C0G 1000pF caps the other day, and sorted them by putting a
    soldering iron on them while measuring (I know, I know, they weren't
    worth the effort, but it was an interesting exercise). Some of the
    C0Gs changed by about .01%, 100ppm, between room temp and way too hot
    to touch -- that's a lot less than 30ppm/C.

    Cheers,
    Tom
     
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