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Photodiodes: Anode, cathode always uncommitted?

Discussion in 'Electronic Design' started by Joerg, Dec 6, 2006.

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  1. Joerg

    Joerg Guest

    Hello Folks,

    It would be nice to tie a TIA into the cathode to get positive polarity
    output in one of my circuits. That would mean the (critical) RF path
    also needs to be connected to cathode. The anode would then get the bias.

    On all PDs I looked at so far neither anode nor cathode are connected to
    the housing. Is this customary or should I assume that cathode could be
    grounded or have higher capacitance to the enclosure on some models?
  2. John  Larkin

    John Larkin Guest

    Interesting issue. I have here an Appointech InGaAs pin diode. It's in
    a 4-lead TO-18 can, one lead obviously the can itself, one lead
    snipped off, and the remaining two leads the pin diode, floating from
    the can.

    Capacitance a-k is 2.34 pF

    A-can is 1.12

    K-can is 1.19

    so at least this one is pretty symmetric.

    But nobody is consistant here, not even on pinout. Lots of laser
    diodes are soldered to the can inside, for heat sinking I suppose.

  3. Strange, the ones I have used, that have a metal case, have the case
    connected to the cathode. (This is often a pain, since you then need a
    negative supply to reverse-bias the diode, or else float the case).
  4. Joerg

    Joerg Guest

    Interesting. Can you share a part number there? With the cathode
    connected to case you'd be ok since the classic way of biasing is AFAIU
    connecting the cathode to a positive supply and running the anode into
    the TIA input. So the anode needs to be really low capacitance.
    Unfortunately that results in a negative voltage swing.

    Of course, now the case would be on a DC level (but RF ground) and that
    source thus needs to be protected against shorts. So it's not the fiery
    scenario that results from wedging your wedding band between GND and a
    car battery terminal.
  5. Joerg

    Joerg Guest

    That would be nice but John Devereux mentioned in the other post that he
    has some where cathode and case are joined.

    I assume it's the cathode that's grounded on those. Otherwise driving
    them would be a bear.
  6. Sure, e.g. Hamamatsu S5821, S2386, Centronic OSD15 etc.

    Ah yes, of course this must be why they do it that way around. My
    applications have been relatively low frequency (<1MHz).
    Still inconvenient though!
  7. Joerg

    Joerg Guest

    Thanks, John. Hamamatsu is a main contender so if they connect the
    cathode to the case then I better not use that as my signal tap.
    For RF that positive bias is fine because you can run into an RF amp
    that is biased properly. But it's a drag if you have to measure DC also.
    Either bias the TIA upwards which for a broadband version is a white
    knuckle ride or provide a negative supply. Oh drat, so it's negative
    supply then for me.
    Yes, especially for stuff that has to go down to DC.
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