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interference and noise question

Discussion in 'Electronic Design' started by Boris, Nov 5, 2006.

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  1. Boris

    Boris Guest

    Hello

    I'm working on a project using a load cell that needs to amplify a low
    signal with a very large gain. I have, for exemple, 2 stages of gain.
    The first(using an instrumentation amplifier) the gain is set to 500x.
    The second gain (using an opamp) is 20x. The problem is that if I
    increase too much the gain, there are a lot of interference. If my hand
    gets closer to the device it increases 1V, and Im supposed to have a
    precision circuit! I've read about ground loops and stuff, but is there
    anything someone can suggest to have a high gain unity without
    interference? Is some kind of isolation needed?
    Thanks a lot
    Boris
     
  2. Eeyore

    Eeyore Guest

    What sort of interference ?

    I hope the first stage is differential and symmetrical impedance on both inputs
    ! More info is required such as the device and circuit configuration.

    Graham
     
  3. 1) how far is the cell from the amplifer (INA)
    2) is the cable between the cell and INA screened?
    3)the INA is in a differential configuration, is it?
    4) what bandwidth do you need?


    martin
     
  4. John  Larkin

    John Larkin Guest

    That's a huge amount of gain. If the load cell has full-scale output
    of 5 mv/v, and you excite it from 5 volts, you should get +-25 mv
    full-scale, so a gain more like 200 might be reasonable.

    Lots of instrumentation amps are sensitive to ambient RF pickup. The
    bipolar input junctions make good RF detectors, and rectify ambient
    radio stations and such into DC offsets. Offsets caused by
    hand-waving, or grabbing leadwires, suggests RF. Your board should
    have a solid ground plane, be in a sturdy box (for thermal reasons,
    too) and all signal and power paths should be well EMI filtered.

    You can expect some hum pickup, and lots of vibration noise, so a slow
    integrating a/d converter, or lots of signal averaging, is usually
    needed for load cells.

    What's your physical construction like?

    I could post a known-good load-cell conditioner front-end schematic to
    abse if there were huge demand.


    John
     
  5. I'd love to see the schematic, as I havent really done much with
    modulated DC :)




    martin
     
  6. joseph2k

    joseph2k Guest

    Use twisted shielded pairs both to and from the load cell, if necessary
    drive the shield from the load cell with the common mode signal. Use
    separate shielded twisted pairs. Also why such high gain (10,000x)? (100x
    to 1000x total gain is usually enough)
     
  7. Eeyore

    Eeyore Guest

    80dB of voltage gain is not unknown for microphone preamps and they can be made
    with excellent interference rejection.

    Graham
     
  8. You should be using shielded cable for all wires going to the load
    cell.

    What kind of bandwidth do you need? If it's less than 60Hz, you can
    roll off the response of the amplifiers with some capacitors, then
    you'll lose all the power-line and higher related noise.

    You'll also need really quiet power supplies and some filtering right
    at the amps.
     
  9. John  Larkin

    John Larkin Guest

    Sure, gain is cheap. Microvolt DC precision and PPM linearity and
    stability are not.

    If you condition a good load cell right, its stability and linearity
    are astounding. Put some, roughly, half-scale weight on a good load
    cell system and note the reading. Then put a different one on the
    scale, ditto. Now weigh both together and note the indicated weight.
    If you do everything right, the two small numbers will add up to the
    big number to a few 10's of PPM. Microphones don't work like that.

    Load cell conditioning and thermocouple conditioning are similar
    problems, with thermocouples being a bit trickier.

    John
     
  10. John  Larkin

    John Larkin Guest

    OK, it's up.

    John
     
  11. Eeyore

    Eeyore Guest

    I was referring to the interference rejection part of things.

    There's no reason it should be a problem.

    Graham
     
  12. John  Larkin

    John Larkin Guest

    Oh, with all that gain, make sure the amp chain isn't oscillating.

    John
     
  13. Boris

    Boris Guest

    John Larkin escreveu:
    Hello John,

    First of all, thanks for helping.My first experiment using this load
    cell had a gain of approximately 500. The reason I increased the gain
    is because I could only notice a small change in the signal when I
    tested it. It is for use with a impact machine, so, the load cell is
    made up of iron with some other hard material so that it almost
    doesn´t deform.After increasing the gain I could notice the signal as
    the mass impacts the object.

    I don´t have much experience with these projects so I´ll try to
    answer your questions the way i´m doing and the way I believe is
    right(but it might not be).

    The signal caused by the impact lasts approximately 50ms, so I assume I
    need a large bandwidth. I put a filter after the AMP in a 40Khz cutoff
    frequency. I have a 60Hz noise disturbing my system, thats the main
    problem regarding filtering in my opinion. If I get rid of that noise,
    It´ll be amost good.
    I´m afraid I lose the signal(it gets filtered) if I´d put a 30HZ
    cutoff frequency, thats why I haven´t decided what filter to use.

    My physical construction is a breadboard, but I´ll change to a printed
    circuit board as soon as possible.

    You said your schematic is up, how can I see it? Could you send me the
    link?

    Thanks a lot!

    Boris
     
  14. John  Larkin

    John Larkin Guest

    It's in the alt.binaries.schematics.electronics newsgroup. If you
    don't get the binary groups, email me. jjlarkin atsign
    highlandtechnology dotty com.

    A more sensitive load cell, better matched to the mechanical load,
    would help. But with proper shielding and signal conditioning, 60 Hz
    pickup should be far below the thermal (random) and vibration noise
    floor.

    John
     
  15. Rich Grise

    Rich Grise Guest

    I'm just a techie, but would it make any sense to swap the gain stages?
    i.e., the 20X first, then the 500X?

    Thanks,
    Rich
     
  16. Eeyore

    Eeyore Guest

    In which case you haven't paid the necessary attention to screening and signal
    balance.

    Can't you post a schematics in alt.binaries.schematics.electronic or on one of
    the free picture hosting sites ?

    Graham
     
  17. Eeyore

    Eeyore Guest

    Usually not.

    Graham
     
  18. John  Larkin

    John Larkin Guest

    Ideally, about equal gain per stage gives best bandwidth and
    stability... 100x100 maybe. But low frequency stuff like this isn't
    all that picky.

    But with 10k gain and 10's of KHz bandwidth, overall GBW is in the
    hundreds of MHz, so a clean layout and a good ground plane are
    prudent.

    John
     
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