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Photodiode Amplifier

Discussion in 'Electronic Design' started by Guy Macon, Feb 20, 2004.

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  1. Guy Macon

    Guy Macon Guest

    I am working on a circuit to detect a very faint spot of IR
    light on an X-Y PSD photodiode. I can make the spot as
    bright as I want, but brighter equals more IR LEDs and a higher
    cost. Response time is not an major issue; I would like to
    modulate my source in the 1khz to 10khz range, but will go
    much lower if needed.

    I made two test PWBs with short leads, ground plane,
    and guard rings, using two opamp stages as follows:

    My first stage is an opamp in transimpedence (current to
    voltage) mode. "+" is gnd, and "-" has a photodiode to ground
    and a large feedback R and small feedback C from the opamp output.
    (I have no bias on the photodiode now, but will be experimenting
    with using a small (0-50mV) bias later).

    My second stage is a standard inverting opamp voltage gain stage.

    One of my PWBs uses single opamps (+2, -3, out6, +vcc7, -vcc4)
    and the other uses a quad (+1/7/8/14, -2/6/9/14, out1/7/8/14,
    +vcc4, -vcc7).

    I am running tests with the best opamps I know of and with the
    cheapest opamp I know of. I am looking for the best tradeoff
    between receiver cost and transmitter cost.

    For the singles, I am using a Linear LT1792 for the first
    stage and a LT1793 for the second. I chose them because
    the LT1792 has (0.8fA/sqrtHz) of current noise and the LT1793
    has (4.2nV/sqrtHz) of voltage noise. For the quad I am using
    an LM324 because I can get them for six cents each.

    That pretty much covers the high-performance end and the
    cheap end. Now I need to try a few opamps that are between
    the two extremes. Can anyone recommend a few parts that
    have a good tradeoff between low current noise, reasonable
    voltage noise, low cost, and which match one of the two
    pinouts I am using?
  2. Guy Macon

    Guy Macon Guest

    Thanks! Advice much appeciated.
  3. John Larkin

    John Larkin Guest

    LM324 is maybe the worst opamp still in production (anybody know of a
    worse one?). Lots of crossover distortion and stuff.

    LF347 quad jfet amps are nice... fast and very low current noise. I
    show 34 cents as our inventory price.

    An LM1458 (basicly a dual 741) is very cheap.

  4. Jamie

    Jamie Guest

    a TL080 is a good op amp, cheap.
    its a JFet input with bipola out with
    lots of gain over the convensional types.
    TL080 = single op, TL082 a dual type.
    and TL084 = quodd...
  5. Modulating the light is an excellent idea.
    Do a lock-in then. A few AC coupled amplifiers
    with sufficient high gain plus a narrowband
    filter. I can recommend the LT1128, with 2 stages
    you can get a gain of 10k..100k (80..100dB)

  6. Guy Macon

    Guy Macon Guest

    Worst for what purpose? Low-light / low-speed pPhotodiode amplifiers,
    to take one particularly germane example, never cross over, and thus
    don't care about crossover distortion.
    That's a good one from a price/performance standpoint. Thanks!
    The cheapest LM324D costs 6.7 cents (Digikey) while the cheapest
    LF347 is 19.6 cents. My current project will be using 500,000
    quad opamps per year (or two million singles) so that extra 12.9
    cents will cost us $64,500 (12.9 cents times 20 opamps per unit
    times 100,000 units). (I can get both parts cheaper, but the
    difference won't change much.) That's not a bad price if it
    turns out that the LF347 allows me to take a dollar of the cost
    of my 100,000 light emitter assemblies.

    You can see by these numbers why I care so much about price vs.
    performance. At it's peak, my largest quantity design was selling
    well over 100,000 units per day at a retail price of $24.99.
  7. Guy Macon

    Guy Macon Guest

    It's essential when dealing with low level of light from
    the emitter and high levels of DC and 120Hz modulated light
    from the environment. Another thing that helps a lot is a
    narrowband optical filter at the color of the emitter.
    I am doing that. I can do a good analog filter for a
    dollar or two, depending on how good a filter I want
    to make, or I can use a four dollar DSP and do a FFT
    filter, which will allow me to have a cheaper emitter.
    I am looking into both options, and will choose whichever
    gives me the lowest total cost.
    The LT1128 is a fine part, but at roughly $4 each times
    four opamps (using the LT1128 for the critical first
    stages only and cheaper opamps for the other 16) times
    100,000 units it will cost me about a million an a half
    dollars to get (4.7pA/sqrtHz) of current noise. The
    LT1792 has (0.8fA/sqrtHz) and is a buck or two cheaper
    per part.
  8. The frequency could be higher, and the filters become
    more manageable in terms of capacitor size.
    The two are not comparable.
    While you can have an analog filter operating at
    signal levels below 1mV, that is not the case for a
    digital filter.
    Ok, current noise appears to be the spec then. I was
    more thinking in terms of voltage noise.

  9. Guy Macon

    Guy Macon Guest

    (Does anyone else have a favorite quad opamp? I need to pick
    another 4 or 5 from the near-infinite choices, and I figure I
    might as well start with ones that others here like. My main
    tradeoff is cost vs. current noise, but I will look into any
    part that anyone suggests.)

    The limiting factor is the response time of the photodiode.
    Low light levels call for large area photodiodes (slower)
    running in photovoltaic mode (slower). This puts your upper
    modulation limit in the 1khz to 10khz range. Within that
    range you want to be as far above the 120hz from fluorescent
    lights as possible.

    I am not sure that I understand your comment about capacitor
    size. You can make any kind of opamp filter you want in
    the 100hz to 10khz range with capacitors in the 0.01uF to
    0.1uF range and still have reasonable resistor values.
    I know of very few applications where a 1mV signal stays at 1mV.
    Usually it gets amplified to a reasonable level. In my design,
    it ends up in an ADC on a microcontroller or DSP. You still
    need minimal analog filtering with the DSP approach, of course;
    you need a low pass RC for antialiasing, and a high pass RC
    filter insures that the later stages don't saturate because
    of the low-level 1-10khz AC modulation signal riding on the
    high level DC and 120hz background signal. The tradeoff is
    in what I do after the signal is goes through the minimal HPF
    and LPF and is amplified to a few volts; do I run the signal
    through a multipole analog bandpass filter or do I run the
    signal through a digital filter using a DSP? I can do a good
    analog filter for a dollar or two, depending on how good a
    filter I want to make, or I can use a four dollar DSP and do
    a FFT filter, which will allow me to have a cheaper (less
    bright) emitter. I am looking into both options, and will
    choose whichever gives me the lowest total cost.
  10. John Larkin

    John Larkin Guest

    Will this thing have an ADC and a uP to control it? If so, why not
    just turn the light source on/off under software control and digitize
    the optical sigs, making a synchronous lockin amplifier out of the
    whole thing? The on/off cycles wouldn't even need to be strictly
    periodic... you could, accidentally or by intent, wind up with a
    synchronously detected pseudo-random-period-spread-spectrum lockin
    thing. You can do some amazing things with one of those 75-cent uPs
    with built-in adc/mux. You only have to pay for code once.

  11. Guy Macon

    Guy Macon Guest

    But of course! :)
    Alas, my emitter is fixed, my detector is mobile, and my detector
    sees a spot that the emitter put on a ceiling. I can't say more
    without revealing too much about what I am designing, so I apologize
    for only describing the subsystem rather than giving the big
    I think I can still do this. I just need to detect the on/off
    cycles and lock in to them.
    SEVENTY FIVE CENTS!?!?! I don't need a *supercomputer*!!! <grin>.
    At Mattel, we paid between five cents and thirty cents for a uP,
    plus a few thousand for a ROM mask.
  12. John Larkin

    John Larkin Guest

    Then why use a $4 DSP?

  13. Guy Macon

    Guy Macon Guest

    John Larkin says...
    Because it may be cheaper.

    With a DSP I can do sophisticated FFT-based filtering.

    With better filtering the system works with a worse SNR.

    With a lower SNR requirement the light emitters can be dimmer.

    Dimmer light emitters are cheaper.

    (But are they enough cheaper to pay for the DSP?)

    With a cheap uP I need to do analog opamp-based filtering.

    With analog filtering the system needs a better SNR.

    With a higher SNR requirement the light emitters must be brighter.

    Brighter light emitters cost more.

    (But does the cheaper uP save enough to pay for them?)

    Any suggestions for the cheapest way to project a modulated
    infrared dot (size doesn't matter with a PSD) on a wall are
    welcome. Right now I am using the IR LED with the most light
    for the lowest price, and using multiple IR LEDs to get a
    brighter dot. How many? I don't know how many yet.
  14. Vishay makes some narrow angle IRLEDs that have a pretty high power
    output for very little money (TSAL6100). I an interested on your
    system optics. Have you considered a plastic fresnel lens that is
    made of visible blocking material? You can get a good light
    collection efficiency and eliminate an extra filter layer.
  15. John Popelish wrote...
    The LED's 10-degree angle isn't so unusual. But its
    1A pulse rating is rather nice.

    - Win

  16. Guy Macon

    Guy Macon Guest

    Thanks! mouser has them for 10.6 cents in Qty 2000 (I emailed for a
    Qty 100,000 quote). That part is much brighter than what I was
    thinking of using.
    I was figuring on seeing if combining the lens and filter would
    lower my cost, but hadn't considered a fresnel. Good idea.
    Avoids large filter material thickness variations. With a nice
    wide spot (PSDs care about total light, not how bright one part
    is) optical distortion at the ridges shouldn't be a problem.
  17. I ran a bunch with a half amp pulse of 100 us repeated every
    millisecond for several days and saw very little change in the output
    power. I have an array of 12 of them aimed at a single spot about an
    inch from the LEDs that warms my finger. At 5 for a dollar they are
    quite a deal.
  18. You might also want to check out some of the other Vishay choices,
    like this one that has an 870 nm output that may produce a little
    higher response (especially speed) from your PSD.

    All choices:

    With quantities above well 1000, you may be able to get a good price
    directly from Vishay.
  19. Phil Hobbs

    Phil Hobbs Guest

    I would have thought that the price of the position-sensing diode would dwarf
    any of these cost savings. Are you using a quad cell or a lateral-effect
    device? In photoconductive or photovoltaic mode?

    Replacing an expensive position-sensing diode with a couple of small solar
    cells plus a shadow mask ought to save much more money than a dime a circuit,
    no? There's a discussion of this exact thing on P. 364 of my book, which you
    can read free on Amazon if you like--start at
    and search for "364" and then on "94" for a lot more details on p. 94.

    (I tried pasting the links directly, which used to work, but Amazon seems to
    have changed their "search inside the book" service to be session-specific.)


    Phil Hobbs
  20. Guy Macon

    Guy Macon Guest

    Roughly $3.40. (waiting on quote of exact price)
    When you are designing high-volume consumer items, the concept of
    dwarfing other cost savings doesn't apply. If I can shave 2 bucks
    off of the PSD, I will *still* want to shave off a fraction of
    a penny anywhere else that I can.
    Hamamatsu S5991-01 tetra-lateral.
    Photvoltaic. My light source is dim and I don't need fast response.
    I also have my technician setting up a "near photovoltaic" test
    where you apply a small (1 to 50mV) bias.
    As you can imagine, i would very much like to replacing my
    expensive PSD with a couple of small solar cells plus a shadow
    You just sold another copy of your book! :) I will definitely be
    looking into this.

    In your opinion, is your technique suitable for detecting the positions
    of three modulated spots projected on a ceiling by IR LEDs?
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