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Low absorption SMD capacitors

Discussion in 'Electronic Design' started by [email protected], Dec 15, 2006.

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  1. Guest

    Does anyone make SMD polypropylene capacitors? I need a type with low
    dielectric absorption for use in an integrating photodiode amplifer,
    value 100pF - 10nF. 5% tolerance is OK as the product will be
    calibrated before shipping; tempco also isn't much of an issue as we
    can measure the ambient temp and compensate.
    I've built a prototype with a through-hole polypropylene cap which
    works well; now I need to build the production version with all SMD
    parts, as there isn't much space to work with.

    Cost isn't much of an issue; it's a low-volume, high-value product.

    I'm basing my choice of dielectric on this app note:
    http://www.national.com/rap/Application/0,1570,28,00.html

    There is some long-time-constant effect visible with the circuit I have
    built but it's negligible compared with the signal. Just for an
    experiment I tried an X7R SMD ceramic, the dielectric absorption made
    it totally unusable.

    The nearest SMD type I can find is polypropylene sulphide from several
    different manufacturers; they all *claim* low dielectric absorption but
    don't provide any figures. I can't find any comparisons between PPS
    and polypropylene along the lines of the graph at the end of the
    National app note.

    Teflon would obviously be better still but I can't find *any* of these,
    leaded or SMD - have they been banned under RoHS or something? :)

    TIA
     
  2. Look for polyphenylene sulphide capacitors. It has worse drift than the
    polypropylene, but otherwise very good characteristics, and a few types
    are available in SMD. Also try a company called 'Electronic Concepts
    Inc.', who specialise in slightly 'upmarket' capacitors, and were helpful
    to me in the past when looking for something with better characteristics.
    I think they do a polypropylene type.
    The reason for the 'lack', is that the film ceased production a little
    while ago. They actually arranged a run of film specially for these
    capacitors. They also do PTFE capacitors. :)

    Best Wishes
     
  3. Some years ago I found that NPO ceramic microwave capacitors worked very
    well. The ones we used were made by American Technical Ceramics. This
    was a true 16-bit hybrid track/hold amplifier (AD386 if you still have an old
    Analog Devices catalog on your shelf). They provided about 20-40ppm of
    DA error alone IIRC, we were able to get nearly an order of magnitude better
    with some laser trimming (not the cap, but some other compensating stuff).

    We learned some other interesting things during this project as well. They
    may not apply to your design. When we were breadboarding the circuit
    prior to designing the substrae for this chip-and-wire hybrid, we found that
    sometimes the DA was awful, and sometimes not. Ultimately it came down
    to the op-amp packaging! Metal cans were great, plastic nearly as good,
    ceramic (a.k.a. CERDIP) was right out. It definitely wasn't the op-amp,
    (AD744, BTW) we had part from the same wafer lot in all three packages.

    Steve - still at ADI after all these years

    For direct email replies remove the 3-letter food item from the address
     
  4. Guest

    Thanks, I'll look them up.
    One thing which has bitten me already is surface leakage on the
    prototype board (soldering residues etc). I must have got lucky with
    the first one as it worked perfectly. #2 and #3 were totally unusable
    at first, I couldn't scrub the residue off as it would rip all the fine
    wires off the SOT23 transistors I had bodged onto the protoboard. A
    gentle wash with isopropanol improved things a lot, with a proper PCB I
    can give it a proper scrub and then conformally coat it.
    We are trying to measure photocurrents of about 200 pA upwards, so I
    will have to keep a close eye on leakage paths. Is ordinary FR4 PCB
    with conformal coating adequate for this job or will I need to look at
    ceramic hybrids etc?
     
  5. Guest

    OK, thanks.
    Oh dear. This design will be in production for 5+ years so I may end
    up having to do a lifetime buy. At least we aren't talking massive
    volumes here...
     
  6. Guest

    OK, thanks.
    Oh dear. This design will be in production for 5+ years so I may end
    up having to do a lifetime buy. At least we aren't talking massive
    volumes here...
     
  7. wrote...
    Steve is right. I have taken measurements on three brands of NP0 C0G
    0805-size capacitors and found them to be either perfect, or below
    0.1% D.A. over a 100Hz to 250kHz range (this is close to my hp 4192's
    supposed 0.2% measurement accuracy), which is as good as my "pefect"
    polypropylene capacitor measurements. This compares to 5 to 15% D.A.
    that I measured for other ceramic-material 0805 capacitors.

    Steve, is your concern the D.A. of a ceramic package's capacitance?
    You have to clean the board shortly after soldering, then add a
    conformal layer to a dry board after test. FR4 should be OK in the
    region you're using, provided you have appropriate guards. In some
    instances you'll want to add rows of closely-spaced guarded vias to
    intercept the currents that could flow along fibres inside the PCB.
     
  8. You will find that companies who are used to working with low leakage
    boards, will offer low residue cleaning, as a special service. I have some
    boards used for reading a couple of sensors, where virtual grounds around
    the inputs are used to make the input 'see' an extremely high
    characteristic impedance, and the input feeds in through Teflon standoffs
    and connectors, and with properly specified cleaning proceedures it is
    suprising how good FR4 can be.

    Best Wishes
     
  9. Tom Bruhns

    Tom Bruhns Guest

    Others have mentioned NPO caps--AKA C0G. You can get these over the
    whole range you're looking for, and a decade on either side. It's
    possible to get them in 1% tolerance, if you ever do need closer than
    5%. One thing to be careful about, though, is that large physical size
    multilayer ceramics, in any of the various dielectrics, are prone to
    cracking. Recommend you stay at 1210 maximum size.

    Interesting to hear from another poster about the polyphenylene
    sulphide film production. I need to pass that info along to a team
    member who is planning on using them (in part because of the low DA).

    Cheers,
    Tom
     
  10. Eeyore

    Eeyore Guest

    Polypropylene ?

    I heard that polystyrene film was no longer available but polypropylene's widely
    used.

    Graham
     
  11. Guest

    OK, great. These seem to be a lot more readily available in SMD.

    [more snippage]
    By 'guards' I assume you mean a ring around the sensitive nodes
    connected to GND?

    All our boards are made by a subcontractor so I don't have much control
    over the cleaning - some have been fine, some have come back covered in
    white detergent residue and other [email protected] which has played havoc with even
    fairly low-impedance circuits. If I scrub them myself with a
    toothbrush and isopropanol after they've been sitting on a shelf for a
    week will this be OK or does the contamination leach into the FR4
    itself?

    I have used the LT1112 (plastic DIP package) for the prototype, it
    seems to work fine but I note from the data sheet that the worst-case
    input bias current and offset voltage could be a problem. In one sense
    it doesn't matter because the photodiode dark current will usually
    dominate, and I subtract this out by taking a dark measurement at the
    same integration time immediately afterwards, but if there is a large
    enough negative bias current the ADC will clip at zero and this method
    won't work.
    Another 'oddity' is that the SO8 package of this device (which I'm
    planning on using for production) has significantly worse specs than
    the PDIP - any ideas why? I always assumed the die was identical
    between different packages.
     
  12. Yes, that was it precisely. Parts from the same wafer lot in different
    package types showed different amounts of dielectric absorption. We
    never bothered to figure out if it was the ceramic in the package, or the
    glass frit seal material, but parts in the CERDIP package showed much
    worse DA than in plastic or metal cans. Ultimately we didn't really care
    as the final product was a chip-and-wire hybrid on a single-layer thinfilm
    substrate with laser-trimmed Nichrome resistors.

    We did also see a difference in cap materials. NP0 were generally good,
    but the best ones were the ATC microwave caps designed for really low
    ESR at superhigh frequencies. I can't remember if they actually specified
    DA, but those caps were quite good. At $1+ each they should have been!

    steve
     
  13. Tom Bruhns

    Tom Bruhns Guest

    Guards are not necessarily connected to ground (optimally), but to a
    potential which is driven from a low impedance and follows the node
    (trace; vias) that you want to protect. If the node you want to
    protect is a virtual ground, then a grounded guard is perfectly
    appropriate. But if you want to guard the + input of an op amp that's
    used in the non-inverting configuration, you can guard that node with
    traces and vias that are connected to the - op amp input. You have to
    beware of capacitance to ground (or to other nodes) from that guard
    trace, though, because that can affect the response. You can also use
    an op amp buffer to drive the guard. Using a guard reduces both DC
    leakage currents and effective capacitance.
    Don't have much control over your subcontractor? Ouch! Complain, and
    if they don't solve the problem, fire them and find one that will
    listen to you. Your situation may prevent doing something that
    drastic, but at least give them a good piece of your mind if they
    deliver [email protected] We've found that our contract manufacturers don't always
    get it right the first time, but they do want to deliver a product that
    meets our needs, and they do listen to us.
    ....
    Cheers,
    Tom
     
  14. Tom Bruhns

    Tom Bruhns Guest

  15. vasile

    vasile Guest

    I'm asking info directly from the AVX when the parts meet my
    requirements. They answer quite fast. However AVX is expensive (twice
    than Panasonic for example).
    On the application posted, the capacitor's dielectric loss is not the
    only problem as long the photodiode current is *very* small. Keeping
    the board clean around the low leackage points is much difficult than
    founding a good capacitor.

    greetings,
    Vasile
     
  16. Tom Bruhns

    Tom Bruhns Guest

    Hi Vasile,

    But the question is not about dielectric loss or leakage, it's about
    dielectric absorption. See, for example, Bob Pease's article at
    http://www.national.com/rap/Application/0,1570,28,00.html (also posted
    in the basenote to this thread). Bob says he has never seen any other
    articles about the effect, but my uncle was issued a patent on a method
    to get around the effect, long before Bob's article came out.

    I have an ongoing experiment involving the room-temperature
    self-discharge time constant of polyester (mylar) and polypropylene
    caps. Last time I checked, the polyesters managed between 5 and 10
    years, and the polyprops came in at about 50 to 100 years. Indeed, it
    becomes diffidult to guard traces with a voltage that's close enough to
    the voltage on the guarded trace to get leakage currents that low.
    Even a 100uV offset, with 10^10 ohms, comes out 10fA I believe, and
    it's easy to end up with less than 10^10 ohms, especially if the
    humidity is high.

    Cheers,
    Tom
     
  17. joseph2k

    joseph2k Guest

    Wow, that triggered an old memory. Back in the day ~20+ years ago as a test
    engineer i had to create fixtures for repeatable mass testing of SMD
    capacitors from very large (0.250 inch by 0.350 inch) down to about 0603
    size. It turned out to be precision construction (less that 0.001 inch
    tolerances) and air insulated.
     
  18. I heard there used to be problems long ago when they changed EPROMS
    from sidebrazed to CERDIP. The sealing glass released quite a lot of water
    during fritting and the chip passivation did not take that into account.

    regards, Gerhard
     
  19. Some articles on DA are on

    <http://waltjung.org/Classic_Articles.html>

    regards, Gerhard
     
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