Connect with us

Cell phone noise into opamps

Discussion in 'Electronic Components' started by Joerg, Nov 13, 2007.

Scroll to continue with content
  1. Joerg

    Joerg Guest

    Hello Folks,

    In a really sensitive sub-audio application we are experiencing cell
    phone pickup. It got a lot worse when switching from the LT1112 to the
    LM833 in order to lower the sub-audio noise (and yeah, reduce cost...).
    Anyhow, this surprised me because the LM833 doesn't even have the input
    protectors like the LT1112 so the chances of something clipping and
    demodulating should be lower. How could this be? Older process with
    larger patterns and thus more pickup loop area?

    The challenge here is that bypassing is all nice and dandy but
    capacitors always have a residual inductance and at cell phone
    frequencies that begins to matter. Cell phones are particularly nasty
    because anything that gets AM demodulated produces this rat-tat-tat
    pulse pattern between phone and cell tower. Shielding is going to be a
    major pain in the neck in this application.

    In case anyone wants to look at the datasheets:
    http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1010,C1764,P1364,D1714
    http://www.national.com/ds/LM/LM833.pdf
     
  2. In a really sensitive sub-audio application you use LM833 despite of all
    your complaints :)
    It could be some resonance affected by the parasitics of the opamp.

    VLV
     
  3. John Larkin

    John Larkin Guest


    Yup. The b-e junctions think they're crystal sets. I've sold about
    4000 thermocouple temperature controllers because the Ox**** units
    rectified RF so well you could shut them down with an old GenRad
    generator from across the room. Well, that and the fact that they lost
    the source code.

    Ferrite beads and small caps to ground help enormously, and won't
    affect baseband performance. I've also observed that such rf
    sensitivity is often associated with sharp resonances in the wiring
    and packaging. A well-placed bead or figure-8 ferrite, in a cable, can
    kill a resonance nicely. We picked up another 20 dB by running our
    thermocouple wiring, from a connector to the pcb, through a fig-8
    (dual-hole) bead.

    One quick fix would be to dump in a jfet opamp. Some of them are very
    good.

    I saw an appnote somewhere that graded various opamps on RF
    sensitivity, but I can't remember where.

    John
     
  4. But you will have to.
    Metal case
    All inputs and outputs including power.
    Do not blame it on the chips.
    (Well there may be some chips.....)
    :)
     
  5. Tilmann Reh

    Tilmann Reh Guest

    Ack. However, the JFET types often tend to be more noisy.

    I made the experience that it's often good to place a small capacitor
    between the two inputs of the OP. That doesn't affect baseband behaviour
    much, but effectively short circuits the RF before it can be rectified
    by the junctions at the OPs input.

    Tilmann
     
  6. Guest

    Basically, if you hit the NPN inputs of a bipolar op amp with more
    than about 26mV of ptp noise at a frequency too high for the output
    put to follow, the inputs follow the upper edge of the envelope of the
    noise, because you've got out of the region of linear response.

    For FET input op amps, the corresponding level is about a volt, which
    is what John is talking about. Low noise FET-input op amps are not
    cheap.
     
  7. Joerg

    Joerg Guest

    Which complaints? Yes, we might go discrete at some point by I think the
    LM833 is a nice amp. Just somewhat fussy when RF is around.
     
  8. Joerg

    Joerg Guest

    Hmm, maybe I could brain-wash the b-e junctions ...

    We are pretty much capped and beaded out already on this one. It helps,
    but it ain't enough.

    As Tilmann pointed out those are too noisy at low frequencies.

    That would be great to have. If I find it via Google I'll post. However,
    in our case we are pretty much stuck because only a handful of opamps
    fulfill the noise criteria. Now I wish there was more LF noise data for
    JFETs because they don't have an engaged b-e junction but are a few
    volts away. Or MOSFETs but there are definitely no LF noise specs for
    those. Maybe I'll start a separate thread, see which FETs people think
    are the best contenders, then buy all those and try one after the other.
     
  9. Joerg

    Joerg Guest

    Not cheap would be ok right now, for this application. We can always
    worry about going discrete later. Got one in mind that is in the
    <10nV/rtHz class and has a farily low 1/f knee?
     
  10. Joerg

    Joerg Guest


    It's not completely possible in this app. Maybe we can find a JFET or
    MOSFET that's low noise instead.
     
  11. Joerg, I find this difficult to answer.
    It seems to me however, if I make something with say -80dB noise level,
    that that should stay -80dB, even when somebody puts a cellphone on top of it.
    Else what good is a noise spec?
    We will only see more and more use of the spectrum, Wimax is coming,
    the higher bands will be used too, EU is working on an "EU wide' spec
    that would leave much more space for unlicensed applications, and give complete freedom
    for the modulation type... Expect at lot of RF.
    http://www.heise.de/newsticker/meldung/98895
     
  12. John Larkin

    John Larkin Guest

    In the signal path?

    John
     
  13. Joerg

    Joerg Guest

    Yes, and also in the supply path.
     
  14. Joerg

    Joerg Guest


    I know. That's just the problem, here we have to deal with cell phones
    that are just a foot or two away from the amp and shielding is a major
    challenge on this one. So a JFET that could reliably deliver around
    5nv/rtHz at 20Hz would be great :)
     
  15. Joerg

    Joerg Guest


    P.S.: Somehow, whenever I am looking up the old faithfuls that could
    have done it such as the 2SK662 they are discontinued. Man, that thing
    rocked when it came to audio noise.

    <sigh>
     
  16. Didi

    Didi Guest

    I know. That's just the problem, here we have to deal with cell phones
    I would try the OPA627 of Burr-Brown (ok, TI). They don't get any
    better
    than that - the speed should be fine for you, noise too.
    But it is not dual...
    A similar - but somewhat noisier and a bit more than somewhat slower,
    but dual is the OPA2107, perhaps it would do.
    However, the way this sounds I would be pretty sceptic that much can
    be achieved without shielding. The cellphone here drives the power
    supply
    of the TV-set mad from 1 ft... what 80+ dB.

    Dimiter
     
  17. Joerg

    Joerg Guest


    Says 40nV/rtHz max at 10Hz. That's almost like the noise of the toilet
    that I installed a couple years ago (where one of our dogs runs away the
    instant I press the flush button).

    Also, right now I am trying to stay clear of BB parts because they
    appear to have some major issues in getting product out the door.
    Digikey has a few old TO cans left for the OPA627 but I assume when they
    are gone that's it. Not good for something that has to go into
    production soon.


    Similar noise but Digikey has absolutely no stock on this one.

    It can be done but not with anything bipolar. Has to be something where
    all the junctions are well biased, like JFETs. We'll go discrete but
    unfortunately JFET datasheets are rather skimpy. Often they don't even
    contain a noise versus frequency graph.
     
  18. Didi

    Didi Guest

    Says 40nV/rtHz max at 10Hz. That's almost like the noise of the toilet
    The graph says something like 15 at 10 Hz.
    The graph underneath indicates at Rs > apr. 5k the noise is all R...
    If the 627 is too noisy for this application, don't waste your time
    looking for anything better - there is nothing even very close to it
    (meaning fast FET opamp).
    Now that would be surprising. I have been using them since 1991 or
    so and they are still in production...

    Dimiter
     
  19. Joerg

    Joerg Guest

    It doesn't have to be fast. Looks like this board has to be migrated
    over to a discrete design. Oh well, it ain't the first time.

    Have you tried to buy any lately? I mean actually ordering a few
    thousand? I don't know what the issues are but I have met a lot of
    people who were in a bit of a frantic mode because their purchasing guys
    cannot get parts.

    [...]
     
  20. Didi

    Didi Guest

    Now that would be surprising. I have been using them since 1991 or
    Now from surprising that gets depressing. I wish I had orders to need
    the few thousand 627s, though... :).

    I am not familiar with the slow parts (not many FET opamps too slow
    anyway, IIRC), but I would still be surprised if there were any
    FET opamp to match the noise figure of the OPA627.

    Dimiter

    ------------------------------------------------------
    Dimiter Popoff Transgalactic Instruments

    http://www.tgi-sci.com
    ------------------------------------------------------

     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-