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AD8015/MAX3645 Oscillating

Discussion in 'Electronic Design' started by [email protected], Jan 16, 2007.

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  1. Guest

    Hello all,

    I'm still trying to put together a TIA and limiting amp circuit. I've
    now coupled an AD8015 with a MAX3645, however the MAX3645 is just
    oscillating. I've followed the circuit example on the AD8015 datasheet
    http://www.analog.com/UploadedFiles/Data_Sheets/AD8015.pdf page 7, up
    to C14/15. At that point the data lines go straight into the MAX3645
    which drives 50R loads. Can anyone spot anything worng with this? Its
    oscillating with no input at all! I can email the exact schematic if
    anyones cares to take a look or i can upload it somewhere if someone
    knows of any web space.

    I'm begining to lose all faith.....

    Andrew
     
  2. Joerg

    Joerg Guest

    Have you tried taking out C14 and C15 and checked if it still
    oscillates? If it does, is the capacitance of the photodiode higher than
    usual? That might require a wee feedback cap for compensation. I didn't
    read the whole data sheet but was a bit surprised that this isn't
    mentioned in the schematic.

    Don't. The first TIA I ever built was also singing like a soprano until
    I figured out that feedback cap issue. Which, of course, hadn't been
    mentioned in its datasheet either.
     
  3. Guest

    Thanks for the reply - and words of encouragement!

    I've tried lifting C14 and C15 and i've also tried shorting the input
    to ground and adding a 2.2pF feedback cap. But i'm still getting what
    looks like noise but could well be oscillation on the output of the
    TIA. I strapped the feedback cap from the output+ to the input.
    However, i've also now noticed that the MAX3645 part is merrily
    oscillating away! (with C14 and C15 lifted there is no input). The
    output is swinging the full PECL output in a lovely sinewave! Back to
    the TIA however, the noise/oscillation is around 40mV and if i had to
    describe it i'd say it was oscillation with lots of noise on top! Does
    this mean my TIA is doomed even without an input or am i measuring
    badly? I'm trying to keep the earth clip local to the signal but it is
    a lead and therefore subject to rf noise. Someone mentioned a technique
    for measuring small signals with a 50R coax, but i'm not too sure i
    follow that.

    Thanks again

    Andrew
     
  4. Joerg

    Joerg Guest

    If both chips are oscillating you might not have a good RF-tight
    prototype. Hard to say w/o photos. Ground clip probes aren't any good
    here. Switch the scope to 50ohms input, hook up a small coax, cut the
    end and hook a 1k SMT resistor in series to the center conductor. All
    really short. Now you can press the shield directly to your ground plane
    and check the supplies, outputs etc., just not any hi-Z input.

    Try a 10-20pF FB cap as an experiment. Should be clean like a whistle.
    If not you might have to work on your bypass caps (must be SMT).

    Don't give up, it can be done. I just used a THS4021 in a similar
    design. That thing is a real hotrod.
     
  5. LVMarc

    LVMarc Guest


    Andrew,

    Faith is what makes electronics work and airplanes fly! Als when plan
    "A" and plan "B" do not pan out there is always "CD&E" the key to your
    solution. Contact me at to continue the discussion

    Marc Popek
     
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