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AD8307 log amp

Discussion in 'Electronic Design' started by Andrej Nussdorfer, Jan 5, 2004.

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  1. Hi!

    I recently got hold of a couple of AD8307 log amps and did some
    experimenting.
    Sadly the performance was quite short of the datasheet specifications.
    The dynamic range I obtained was only about 55dB (instead of 80+dB).
    As long as the amplitude of the signal is above 5mV the amp works fine
    (quite per specifications) but as soon the signal drops below 5mV the
    response gets very non-linear. The circuit follows closely the eval
    board schematic with all the jumpers open (with the exception of
    enable, which is tied to +V) and can be found on
    http://www.fe.uni-lj.si/jade/dump/detect_schematic.jpg. The strange
    thing is that both amps I have work the same way so I guess there is
    something I am missing (and quite badly it seems).

    Any suggestion would be appreciated!

    Thanks. Andrej Nussdorfer
     
  2. LogAmps are quite difficult what the layout and the supply concerns.
    This 8307 does DC to 500MHz. Since the supply rejection at these
    frequencies is not that great anymore, you have to be sure the supply
    is clean. You're not using a switchmode power supply by chance ?
    The layout is a doublesided or a 4 layer pcb ?

    Rene
     
  3. I read in sci.electronics.design that Rene Tschaggelar <>
    Log amps also pick up general EMI stuff in the vicinity, especially AM
    radio, in my limited experience. Try putting your board in a diecast
    box.
     
  4. Hi!

    Thanks for the answers! As far as the power supply is concerned it is
    a linear regulator (LM7805) and it is loaded with blocking capacitors.
    I quite understand that the layout can be a major factor at higher
    frequencies, but all the experimenting I did was with signals below
    10MHz. I guess I can only try and put the whole thing in a metal box
    (as suggested).

    Thanks again!

    Andrej Nussdorfer
     
  5. Yes, but any ambiant noise up to 500MHz will be received by the AD8307, and
    that's easy to getsome mV of noise...

    Cheers,
    Robert Lacoste - ALCIOM : The mixed signals experts
    http://www.alciom.com
     
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