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Photodiode case

Discussion in 'Electronic Design' started by WhiteDog, Oct 27, 2007.

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  1. WhiteDog

    WhiteDog Guest


    I am working on a wideband photodiode receiver for a laser ranging
    system. I have a bootstrapped photodiode plus transimpedance amplifier
    that has a BW of about 75MHz. My signals of interest are in the
    3MHz-6MHz region and the 35-50MHz regions. The whole thing (PD plus
    amp) is inside of a stainless steel box that provides E-field
    shielding (but not much H-field, I fear). I know stainless was not the
    best choice for HF, but there were other factors involved. I am trying
    to get that changed now.

    My question is regarding what to do with the case of the photodiode
    (an InGaAs PIN). The case of the photodiode is providing some strange
    coupling effects. The PD case pokes out of the stainless enclosure
    that contains the PD front end amplifiers, etc. The PD case is NOT DC-
    coupled to anything inside itself (the TEC cooler, thermistor, or the
    PD substrate itself).

    Initially, I had the PD case mounted on the enclsoure itself. The
    thinking here was that the PD case plus the enclosure would form a
    Faraday shield. This was apparently a fairly bad idea, as the
    enclosure apparently capacitively coupled to the PD, and it created a
    feedback path from the amp output to the PD itself, and I had in-band
    ripples in the response at the higher frequencies.

    I tried driving the PD case with the bootstrap. No good.

    I currently have the PD case attached to the 'ground' reference near
    the transimpedance amp with a length of copper braid. This seems to
    have the best results, but still not ideal

    Has anyone else had experience with doing this? What is the preferred
    way of isolating the PD substrate from the E/H field environment?

    Does anyone else have a sore forehead from beating it against a
    network analyzer ;-)?

    Thanks for any help,

  2. Joerg

    Joerg Guest

    I have always grounded the can directly to chassis resp. the ground
    plane on the circuit boards. Reverse bias it, to gain a wee bit more BW
    and avoid rectification of RF noise. I usually select a (very clean!)
    bias of about 80% of spec sheet limit. If BW is still an issue you can
    cascode to reduce the capacitive load by the TIA. Phil Hobbs explains
    this very nicely:
  3. John Larkin

    John Larkin Guest

    You won't have h-field problems, so any metal box is as good as any
    I usually ground the case, in my case to the pc board ground plane.
    Seems fine.
    It should.

    This was apparently a fairly bad idea, as the
    If the pc board that contains the tia is securely grounded to the
    case, and the pd can is grounded, and the leads from the pd to the tia
    are short, it should work.
    What's the pd capacitance? I'm surprised you need to bootstrap. The
    bootstrap drive may be causing some of the weirdness you're seeing.
    Ground everything!

    It would help if you could post a schematic and some pictures.

  4. John Larkin

    John Larkin Guest

    Reverse bias helps a lot. It not only reduces capacitance, mostly in
    the first few volts, but higher voltages sweep charges out of the
    junction faster. So use all the volts you can.

    RC filter it close to the pd.

    His book is worth having if you're doing electro-optical stuff. It's
    fun to read anyhow.

  5. Joerg

    Joerg Guest

    Yes. But I don't dare to come closer than 80% to the abs max. Which can
    be as low as 5V on modern ones so I usually go for 4V.
    Absolutamente. Costs next to nothing.
    I've got to buy one. Not having read Phil's book before my last design
    almost felt like driving without a license.
  6. WhiteDog

    WhiteDog Guest

    Wow. So many other people withh nothing better to do on a Saturday ;-)
    Thanks for the comments, I will head into the lab later this weekend.
    I had tried just the cascode and reverse biasing, but I only had
    available -5V reverse bias.
    That did not get me the BW I needed. The Cd on the part is about 2.5pF
    at that bias. I could
    not resolve this, as I should have had an r_e on the cascode of about
    725 ohms (should have been
    a BW of about 87MHz).
    Yep, I would do this, plus use a ferrite. Or maybe a capacitance
    I have this, it is quite a good book. The circuit is actually the
    bootstrap plus cascode he covers in the book and the paper, so there's
    the schematics (different op-amp in the transimpedance stage). I've
    also corresponded with him several times for clarification (not that
    he was unclear, I was just being dense). Quite a nice fellow.
  7. Yes, perhaps.

    There's sometimes an issue arising in high-gain preamps, with
    the output stage's coax-driving current return path, and whether
    any of that couples back to the input signals. For example, I
    have made wideband high-gain amplifiers (e.g. for channeltrons,
    etc., with 50MHz bandwidths and G = 5000), and I've found that
    sometimes a single ground stud from the PCB to the box was a
    problem, but ringing (or oscillations) stopped once I had two
    ground studs, one near the input and one near the output.

    With two ground connections to the box it's also helpful to split
    the ground planes, mid-circuit (couple with a differential stage)
    and use the box for the required dc-path connection. This scheme
    prevents any of the high coax drive currents from flowing near the
    input sensor and amplifier.

    You can also add an electrostatic shield soldered to the input
    ground plane, made from thin copper sheet stock, etc.

    Another helpful trick you can use when only one ground plane, etc.,
    is available and such modifications would be painful, is to use a
    floating BNC for the output signal, carefully wiring the ground
    return right back to the output stage's bypass cap and its ground
    reference point. There may be a distant ground return, via the AC
    power ground to a vacuum chamber, or whatever, but the distance may
    eliminate any feedback problems. In some cases the high-frequency
    isolation can be assured with a ferrite clamp around the coax cable.
  8. Joerg

    Joerg Guest

    What is the P/N of your diode, and what opamp are you using? My last
    design only needed 50MHz or so but I got well in excess of 100MHz, sans
    cascode and with -4V reverse bias.

    Pay close attention to the capacitances around your TIA. A stray pF here
    and there can really ruin things.
  9. Joerg

    Joerg Guest

    Use all mounting holes for grounding to chassis. Plate them correctly,
    i.e. nickel (do _not_ tin-plate). For long term performance make sure
    mounting HW and studs are plated compatibly. For proto series unit I
    usually order that stuff from McMaster.

    Split or floating grounds are IMHO a recipe for disaster but it's still
    taught at universities. OTOH that does provide a nice income for me
    because I get to reverse all that. The last one was a week ago, but on
    the way home I got stuck in traffic, big time :-(
  10. John Larkin

    John Larkin Guest

    Why not? If it's designed and spec'd for X volts, why not run it at X
    volts? Confession: sometimes we use schottky diodes and phemts at 2X.

    That will still not be completely "depleted", namely the capacitance
    is still on the curvy part of the curve.

  11. John Larkin

    John Larkin Guest

    This gets 180 MHz from a 2.5 pF silicon pin diode at, I seem to
    recall, -9 volts or some such. I just dumped it into a medium-fast
    opamp as the tia, no cascode or bootstrapping.

    Bootstrapping is likely to cause more troubles than it's worth.

    Overkill. Just a cap would be a better hf ground than any active

  12. John Larkin

    John Larkin Guest

    A photodiode is the ideal floating signal source, so doesn't have
    these problems.

    My little e/o boxes have 4 pems in the bottom of the box, with the pcb
    ground plane solidly screwed to all. We're doing a custom one now
    where the board is screwed to several machined metal bars, with lots
    of screws, and the bar are in turned screwed to the box. These are
    serious electrical and thermal barriers.

    The more you ground things, the happier you'll be.
    NEVER split ground planes. NEVER use multiple ground planes.
    We seem to be philosophically incompatible.

  13. The split ground is _inside_ the small box and is a totally
    appropriate way to control where the cable-driving currents go,
    thank you. Without it, even your nickel-plated contact may one
    day become insufficient, with it, the longterm contact quality
    is _much_ less important, if at all. You don't think extremely
    careful control of the RF current pathways is important in
    high-gain wideband amplifiers? I seriously doubt that.

    As for whether it's taught at universities, I wouldn't know -
    that's across town from me. I'm an engineer, not a professor.

    My suggestion for a floating output BNC with ferrite clamp was
    stated to be for a poor bloke who has to live with what's in
    front of him, and needs a way to get it working. In any case
    it's a more reliable technique than you might imagine. Keep in
    mind it would be employed by the user of the boxes in question.
  14. Joerg

    Joerg Guest

    Well, the client has to pay for another 50 or so photodiodes per system
    if the PS hicks up ;-)
    I've looked at that with a Hamamatsu and a JDS diode and didn't find too
    much change. IOW we didn't need the last few percent in BW. Initially I
    thought about making that programmable but then thought about the
    consequences if someone accidentally steps onto that register.
  15. Joerg

    Joerg Guest

    I've seen it go wrong almost every single time. In theory a split works,
    of course. In reality stuff has to be connected to the individual sides
    and suddenly you've got loop antennas. Even in case of opto electronics
    because the stuff on the PD side of the split must be supplied with
    juice. Now you could do that with fully isolated forward converters and
    stuff but with more than one voltage that gets old. Then again I may be
    a bit biased here because many of my designs must work in rather nasty
    environments. Defibrillators, diathermia gear, elevators and so on.

    IMHO the real education happens at institutes. That's where I learned
    some of the ropes (RF Institute at the RWTH Aachen in Germany). Seems
    not to happen much anymore, most of the young grads I interviewed could
    not even solder (!).

    In a pinch a ferrite can "make it work". I do that, too, but only when
    there is no other option. It's like pain pills, makes the symptoms go
    away ;-)

    The last system with isolated coax connection and ferrites was
    ultrasound and before throwing out the split grounds the guys were
    wondering why the image background "lights up" if someone touches the
    transducer jacket.
  16. Joerg

    Joerg Guest

    But the OP should consult the datasheet first. Some newer PDs have -5V
    abs max.
  17. Joerg

    Joerg Guest

    Shhht! Not so loud. This is 30% of my revenue source. Just imagine a
    body shop when nobody crashes their cars anymore.

    So, on to politics then?
  18. Not to beat a dead horse, but to be very clear, the split ground
    is for the purpose of eliminating any loops, or more importantly, of
    preventing two possible paths for the output return current.
    both planes are grounded to the insides of the small box (the input
    an isolated PD sensor). And I will strongly state, you will NOT ever
    find it "going wrong." Quite the contrary. Ahem, may I assume we're
    talking about the same thing here?
    OK, I'll take that as a compliment for the Rowland Institute,
    if I may. (I'll take whatever I can get.)
  19. Joerg

    Joerg Guest

    Maybe we aren't. If you bolt it all to chassis inside the box you don't
    really have a split ground anymore. Might as well make it one plane then :)

    Yes, institutes can play a major role in teaching young students about
    the real stuff. They might cause grief and even destroy this, that and
    the other thing but the value to society you guys are adding by taking
    them in is huge. AoE is the other huge contribution. I guess I may be
    responsible for sales of a few dozen copies, telling young grads that no
    matter what their professors say "you've got to have this book".

    Academia alone seems unable to do that hands-on anymore. One client
    tried out three grads and finally the forth turned out to be a good fit.
    Imported from Canada...
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