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Suggestions needed for parts selection on a high-speed PIN photodiode circuit — Parallax Forums

Suggestions needed for parts selection on a high-speed PIN photodiode circuit

ElectricAyeElectricAye Posts: 4,561
edited 2012-07-26 13:25 in General Discussion
I had some unused space on a recent PCB I had sent out, so on a lark I hastily filled up that space with a circuit I got from Example #6 of this document:

http://sales.hamamatsu.com/assets/applications/SSD/si_pd_circuit_examples.pdf

I've never tinkered with a circuit like this, so I have no idea what the values for R, Rf, RL should be. And I'm just guessing that the V in the equation refers to the voltage of the amp's power input, but I don't really know. The PIN photodiode I just so happen to have laying around is the following:

http://www.vishay.com/docs/81502/bpv10.pdf

Anyway, I'd like to play with this circuit and I need to order the parts. I'm interested in using the circuit to detect flashes of IR, though I don't really know how intense they will be. At this stage, I'm just goofing around with it, but I'd appreciate some suggestions on some ballpark numbers for the R's and such.

Much obliged.

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-07-24 21:02
    'Obviously a question for Tracy. :)

    -Phil
  • LawsonLawson Posts: 870
    edited 2012-07-25 07:30
    " I'm interested in using the circuit to detect flashes of IR" how short are the flashes? The photo-diode load resistor (Rl) combined with the diode capacitance (Cd) and op-amp pin capacitance (Cin) is the first speed/sensitivity tradeoff in that circuit. Those components form a RC low pass filter.

    Lawson
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-07-25 09:12
    Lawson wrote: »
    " I'm interested in using the circuit to detect flashes of IR" how short are the flashes? The photo-diode load resistor (Rl) combined with the diode capacitance (Cd) and op-amp pin capacitance (Cin) is the first speed/sensitivity tradeoff in that circuit. Those components form a RC low pass filter.

    Lawson

    I'm thinking each pulse could be as short as maybe around 5 micro seconds, each pulse spaced by at least 100 usec. Thanks for the insight into the effective low-pass filter. Any ballpark suggestions for what components I might try?
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-07-25 13:36
    Phil Pilgrim (PhiPi) wrote: »
    'Obviously a question for Tracy. :)

    O Tracy. O, brother, where art thou?

    o_brother_tim_blake_nelson.jpeg
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-07-25 14:02
    Do you have an idea of the light intensity on the PIN diode?

    Duane J
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-07-25 15:06
    Duane C. Johnson wrote: »
    Do you have an idea of the light intensity on the PIN diode?

    Duane J

    Hi Duane, thanks for your interest.

    To be honest, I don't have any idea of the light intensity. I just wanted to put one of these circuits together to play with and I don't even know where to start sizing the components. Is there some way to at least guess what the range of the various R's might/should be?
  • Duane DegnDuane Degn Posts: 10,588
    edited 2012-07-25 16:37
    ElectricAye wrote: »
    O Tracy. O, brother, where art thou?

    FYI, Tracy has been helping me out with my attempt to use a photodiode as a light sensor. Maybe some of his advice to me will help you.

    Those TAOS light to frequency chips sure make sensing light a lot easier than using a bare photodiode.
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-07-25 17:03
    Duane Degn wrote: »
    FYI, Tracy has been helping me out with my attempt to use a photodiode as a light sensor. Maybe some of his advice to me will help you...

    Interesting. I'll have to look through all of that excellent material and see what I can learn. Thanks!
    Duane Degn wrote: »
    ....Those TAOS light to frequency chips sure make sensing light a lot easier than using a bare photodiode.

    Yes, I love the TAOS chips, but they suck when you're trying to "integrate" a number of very bright pulses - I think they can saturate during a short bright pulse and give you readings that trick you into thinking that the pulsed light integrated over time is smaller than it actually is. See my old whine:

    http://forums.parallax.com/showthread.php?133699-How-fast-bright-flashes-can-cause-errors-in-the-TSL230-Light-to-Frequency-sensor
  • LawsonLawson Posts: 870
    edited 2012-07-25 17:30
    So if we just want to ball-park things and don't need a detailed pulse shape, 1/5uS will give us a ballpark estimate of the needed bandwidth. (200KHz) If we assume a high speed photo-diode the parasitic capacitance should be 10-20pF. This limits the load resistor to a maximum of 40Kohm with higher 20pF estimate. From here, pick the feedback resistors for the op-amp as usual for a non-inverting amplifier. 40Kohm is a fairly low first stage gain for 200KHz of bandwidth. In my opinion circuit #6 is most useful when trying for >100MHz circuit bandwidth. (i.e. Rl = 50 ohms and a voltage feedback op-amp with GHz scale gain bandwidth product or a current feedback op-amp) Circuit #5 is more useful when trying for high sensativity. With a relatively fast FET op-amp circuit #5 should be able to use a 1-10Mohm feedback resistor at 200KHz bandwidth.

    If you truly are trying to measure a pulse, example #13 might be more useful. It's output voltage in proportional to how much energy has hit the photo-diode since the last reset. It'll let you make one ADC measurement after the pulse arrives, instead of having to catch the pulse at it's peak or record the pulse shape.

    Lawson
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-07-25 18:05
    Lawson wrote: »
    So if we just want to ball-park things and don't need a detailed pulse shape...

    Lawson,

    thanks very much for your insights on this. I guess by just jumping into making one of these PIN circuits, I was hoping to force myself to learn what's actually going on inside one. Your comments have been very helpful.

    :-)
  • Tracy AllenTracy Allen Posts: 6,664
    edited 2012-07-26 00:27
    I'd guess too that circuit #6 with the PIN photodiode is meant for relatively strong but fast pulses of current into a relatively low value RL. Hamamatsu has another nice document (attached) which compares construction and characteristics of different types of photodiodes, and one example there shows a direct connection of a PIN diode to a 50Ω coaxial cable and discusses the choice of the component values. Adding the amplifier would allow a higher value of RL as well as gain to drive the cable.

    The circuit is simple enough and allows for experimentation. Go for it! If you have a good 'scope, that will be your readout. Figure the current from the PIN diode is going to be on the order of 1 µA to 100 µA, so RL=1kΩ and an amplifier gain of x10 will give you a signal that the 'scope can see. Point a flashing light (Prop controlled of course) at the photodiode and see what the output signal looks like. Then play with the component values to improve the sensitivity and/or the speed.

    The subject is fraught with tradeoffs of speed, noise, sensitivity etc. etc. The most interesting reference that I keep coming back to is a book by Phil Hobbs, Building Electo-Optical Systems; Making it All Work. It is full of all sorts of lore and practical information and circuits (but bring your own glossary).

    Linear Tech has some interesting photodiode ideas in their circuit collections.
    pdf
    pdf
    Hammamatsu_photodiode_technical_information.pdf
    340K
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-07-26 08:25
    Tracy Allen wrote: »
    ...

    The circuit is simple enough and allows for experimentation. Go for it! ....

    Tracy, thanks so much for pointing me to these references and providing some great clues. It's just the sort of thing I needed to get started.

    wile_e_coyote_flying-1-1.jpg
  • LawsonLawson Posts: 870
    edited 2012-07-26 13:13
    A bit more info to help you along. Compensate Transimpedance Amplifiers Intuitively gives the basics of making a circuit like #5 stable. I'll also note that the interaction of diode and feedback cap can be useful as it allows the use of non-unity gain stable op-amps for more speed and less noise.

    @Tracy: that Linear tech application note reminds me of www.electrooptical.net/www/frontends/frontends.pdf from the same author as the book you suggested.

    Lawson
  • ElectricAyeElectricAye Posts: 4,561
    edited 2012-07-26 13:25
    Lawson wrote: »
    A bit more info to help you along. Compensate Transimpedance Amplifiers Intuitively gives the basics of making a circuit like #5 stable....

    Excellent stuff. Thanks again, Lawson. But, man, that's some amazing intuition those people must be sportin'.
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