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a low noise level opamp

Discussion in 'Electronic Design' started by huiliu, Dec 11, 2005.

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  1. huiliu

    huiliu Guest

    Can any one tell me where to get a low noise level opamp? It basically
    statifies 10 micro V @ 0Hz -- 5kHz if powered by battery.

    Very thanks
  2. Phil Allison

    Phil Allison Guest

    ** Another Google Groupie.....

    ** If that 10uV is an EIN figure - even a 741 should do.

    What is the source impedance ?

    ........... Phil
  3. Reg Edwards

    Reg Edwards Guest

    There is nothing lower noise than a base-input, small collector
    current, npn transistor.

    If an op-amp has such an input transistor then all well and good.

    For an even better performance - freeze it.
  4. Phil Allison

    Phil Allison Guest

    "Reg Edwards"
    ** Huh ???

    Has Reg never heard of source impedance issues ?

    Never heard of low noise FETs either ?

    Never heard of paralleling BJTs for low impedance, low noise usage ?

    ......... Phil
  5. huiliu

    huiliu Guest

    Sorry, I use it as a output not a input.
  6. huiliu

    huiliu Guest

    It seems ok but not appropriate for my application. and I wanna have a
    output with noise level less than 20 micro V < 5kHz, the load will be

  7. huiliu wrote...
    10uV rms with a 5kHz bandwidth implies an output noise density of
    vn = 10uV/sqrt(5kHz) = 141nV/sqrt-Hz, assuming white noise. This
    is an extremely high noise level you are allowing. Most opamps
    have voltage-noise density, e_n, well under 35nV, and many are
    under 5nV, some are even down to under 1nV.

    Two other noise sources you may encounter are Johnson noise and
    opamp current noise, both involving your feedback resistors. If
    you use resistors below 150k-ohms the Johnson noise, sqrt(4kTR),
    will be under 50nV/sqrt-Hz. And with 150k resistors, if you keep
    your opamp's current noise, i_n, below 50nV/150k = 0.33pA/sqrt-Hz,
    then that source will also be below 50nV. Opamp current noise is
    just shot noise from the input bias current. If the bias current
    is below Ib = i_n^2 / 2q = 0.34uA, that source will be well under
    control. As you know most opamp bias currents are far below that.
    If you use a JFET opamp, that term will disappear entirely.

    If you have four 50nV noise sources, you'll have 100nV of noise
    density, which is 7uV over a 5kHz bandwidth (the fourth source is
    a placeholder for your signal source or voltage reference). You
    may find that aspect harder to achieve, but if the first three
    terms each contribute under 50nV, you'll be allowed 111nV/sqrt-Hz
    for your signal source, while still meeting your 141nV budget.
  8. Ken Smith

    Ken Smith Guest

    I guess you are calling the following "nothing":

    A medium power NPN transistor running at a modest collector current,
    A low noise JFET such as the IF3601,
    A varactor diode,
    A SQUID,
    A MASER.

    I suspect that we could also add a power MOSFET to that list at low
  9. huiliu

    huiliu Guest

    :) very thanks
  10. huiliu

    huiliu Guest

    the analysis is very very useful, and can you show me where I can get
    some sources on noise analysis? I checked some of electronic books and
    got little information.
    thanks again
  11. Pooh Bear

    Pooh Bear Guest

    There are 2 equations that you are likely to need regularly for noise.

    Where a device has a specified input noise voltage density ( typically
    measured in nV / sqrt Hz ) the wideband input noise *in a badwidth B* is
    simply given by

    Noise ( rms ) = noise_density * sqrt ( B )

    When making noise calculations you are also likely to need to consider
    the effect of thermal ( Johnson ) noise from resistors.

    Thermal noise for a ressitor of value R at an absolute temperature T (
    degrees Kelvin ) over the Bandwidth B ( in Hertz ) is given by.

    Noise ( rms ) = sqrt ( 4 * k * T * R * B )

    Where k is Boltmann's constant = 1.37 * 10^-23

    Also when *adding* multiple noise sources be aware that due to its
    'randomness' noise doesn't add arithmetically. Instead it adds as the
    root of the sum of the squares.

    So for several noise sources N1, N2 N3 etc.....

    Noise total = sqrt ( N1^2 + N2^2 + N3^2...... ) etc

  12. huiliu wrote...
    The Art of Electronics has 38 good pages of noise discussion.
  13. huiliu

    huiliu Guest

    a general but usful one.

    got it:)

  14. huiliu

    huiliu Guest

  15. huiliu

    huiliu Guest


    oh, your book:) my pleasure to enjoy it.
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