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Blown opamps in chaos circuit

Discussion in 'Electronic Design' started by George Herold, May 23, 2013.

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  1. So I’ve got this Chaos circuit.
    https://www.dropbox.com/s/afy4kb63ftdo1ke/chaos2.pdf
    (I just installed dropbox.. I hope that works)
    Basically two multipliers and three integrators (with gain and reset)

    I’ve tested about 20 of them so far and in 3 or 4 of the 20 one or
    more of the opa2277 opamps has been fried. Now all the production
    people wear grounding straps when stuffing a PCB. And we use this
    opamp all over the place and I’ve never* seen any problem. So I’m
    trying to figure out what’s going wrong.

    (Finding the blown opamp in this circuit is a bit challenging.
    There’s no real input and output, it’s just a multi-connected
    loop.)

    Now the opa2277 data sheet warns about too much input current. (must
    be less than 20mA) I’m wondering if the stuffing order of the board
    could be an issue. Could there be some charge on the integrating caps
    that gets pushed through the inputs as the pcb is stuffed?

    For the moment I’m having them put sockets in for the opamps.. at
    least that way it’s easy to change. And the opamps will go in last.

    Any thoughts/ ideas would be most welcome,
    George H.





    *that’s not quite true I had a similar issue with an opa2277 used in
    the PID section of a temperature control board.. these were made by a
    board stuffing house and in two (out of ~100) the opamp was also
    fried.
     
  2. No, it arrives on my desk for testing already blown. I can't make a
    working circuit fry the opamp. (I tried all sorts of switch flipping,
    power on /off cycles.. no problem.)
    Well S1 resets the 'thing' You can then watch how the signal changes
    with time from a given starting point.
    SW1 just changes the time scale for the circuit. It's easier to see
    on the 'scope with a fast time scale, but slower has less noise and
    it's easier to find the period doubling points.

    One thing that does stink about digital 'scopes is the X-Y mode.
    Grin... It was easier just to dump it out of eagle than to try and
    redraw it.

    George H.
     
  3. Yup Eagle, Different people add different parts to the libs and end
    up calling things whatever they want. I doesn't bother me... but heck
    I use i for sqrt(-1).

    "Damn it Tim, I'm a physicist not an engineer!" (Channeling Deforest
    Kelly :^)

    George H.
     

  4. It's claimed to be able to withstand +/-30V differential input voltage
    (1K internal series resistors), but nothing else leaps out.

    (I assume you've got a fixture or something to guarantee the +/-15
    isn't mixed up.. there are six different ways to connect 3 power
    supply wires, and not all of them are benign- you're also close to the
    36V abs max supply limit)

    If nothing else comes up, maybe consider back-to-back diodes (eg. one
    BAV99 each) from the inverting inputs to ground.
    Seems unlikely.
     
  5. P.S. Also suggest you check where the op-amps came from and make sure
    that they were not returns or otherwise in question.
     
  6. Grin... I guess I'm the fixture. (potted into my chair perhaps.) I'm
    the first person to supply power to the boards and I always give them
    a visual inspection first... power correct, and check that any tant's
    are in the right way.
    ?? Do you mean back-to-back zeners? Or does back-to-back mean in
    parallel?
    I could do that on the integrators.
    Everything seems unlikely. I thought maybe we got a bad lot of IC's
    and tested a bunch from the tube with the same date code... they were
    all fine. We use this opamp in ~10 different instruments and I've
    never had a problem.

    Thanks for the input,

    George H.
     
  7. Hmm, Thanks I hadn't thought about the other end of the cap. (the one
    tied to inverting input.) So hitting the reset causes the inverting
    input to have a spike (pulse) that's mostly about 1 volt but sometimes
    up to 2 volts. (This is with the circuit configured for the biggest
    voltage output... again in the 1-2 volt range) Shorting the opamp
    output to ground is not that much of an issue.
    Sorry about the NC/NO switch screw-up. The switch has to be open for
    the circuit to run.

    George H.
     
  8. Grin.. I checked a bunch, they were all fine. Most likely came from
    newark. I got some bad 79L12 volt reg from Jameco... a light
    gray?
    I don't go there any more.

    George H.
     
  9. Jamie

    Jamie Guest

    Youi're leaving the caps open, allowing them to charge to what
    ever.

    It's been a while since I've looked at the specs on that op but
    something tells me you could be over doing it on the input, depending
    on what the other input is doing...

    Personally, I would put high value bleeder R's crossing the switch, at
    least this way the caps are more than likely to be close to the existing
    value when switched in.

    You should be able to select proper values that won't conflict with the
    current values selected. 1 MEG. 5 MEgs etc..

    Jamie
     
  10. Yeah! I've been swapping 'em in and out on my bench (no wrist strap
    most of the time.) and hooking up the power backwards every once in a
    while. And never any problems.

    So all of the time it's IC2 (U2) the Y opamp that has been fried, but
    once the Z opamp was also fried. The Y opamp does have the biggest
    feed back R.

    I'm done testing the latest batch, there'll be more coming in a few
    weeks or so, waiting on a part. I'm guessing it won't happen again...
    Some one time ESD event that took out a handful? (doesn't explain why
    it's IC2?)

    George H.
     
  11. P E Schoen

    P E Schoen Guest

    "George Herold" wrote in message
    It is generally good practice to use a resistor to connect an op-amp input
    to ground. The value should be similar to the impedance of the other input
    circuit, so for IC1A through IC3A about 5k would be good. This is mostly to
    balance the offset voltage due to input current, but it may also avoid
    having high current spikes through the input protection diodes to the supply
    rails. Another source could be the direct connection from the AD633s to IC2B
    and IC3B. Also, as others have pointed out, the reset switch may cause a
    current surge from the capacitor discharge. A 10-100 ohm resistor might be
    wise.

    I use the OPA2277UA (SOIC8) in my Ortmaster product and I have not had any
    failures.

    The spec sheet indicates that the inputs have internal 1k series resistors
    for the diode clamps, and also bias cancellation circuitry which makes the
    bias cancellation resistor unnecessary. But I still think it's a good idea.

    Paul
     
  12. So the opamps arrive blown. (or blow on the first power up... but
    never on any power up after that once they have been replaced.)

    Not much room for more parts, and adding those R's would 'screw-up'
    the math a bit.

    I've done several switched cap. circuits, I've never had a problem.

    Say, how big a voltage can a 1uF film cap get with one side open?
    (would it be different for a ceramic cap?)

    George H.
     
  13. P E Schoen

    P E Schoen Guest

    "George Herold" wrote in message
    IC2 is the op-amp that has its output connected to the reset switch to GND.
    It may be able to withstand a short to GND, but it seems like poor practice
    to short an output for reset.

    Also, a high precision low power Op-amp is likely optimized for other
    parameters and not so much for ruggedness.

    Paul
     
  14. Yeah I know... I had a different opamp in there to start with and then
    was confronted with this big DC offset. (note the trimmer pots on the
    multipliers*) The circuit has a gain of ~200 or so, and a millivolt
    of DC offset was a problem. This opamp has more noise than Phil H
    would like, (8nV) certainly part of that is the 1 k ohm R's in the
    input protection circuit.

    George H.


    * They were added after the first spin, I never get my pcb's right on
    the first try. (I haven't used a trimmer pot in more than ten
    years.)
     
  15. Guest

    From the other comments it really sounds like bad parts. But, if not,
    is there any possibility of ESD via the body of SW1? Do any of the
    "good" units' U2's show increased Vos? And, on the bad units, are the
    inputs fused/shorted/open, etc?

    The answers to those sorts of questions might narrow the
    possibilities.
     
  16. Hi James, I'm afraid I don't have much good data. Each integrator
    has a gain of two section in it's other half. And for those I can
    look at input and output to see if they agree. In two of the cases
    the output was railed. (I don't recall which rail.. sorry) Opamp was
    riped out, replaced and no problems. (I'm getting fast at clipping,
    pulling leads, solder sucking, cleaning and reinstalling... Through
    hole parts are easy that way.)

    For the other two (of four) bad ones I just ripped out U2 and put in a
    new one, in one case I had to replace the Z opamp also... maybe U2
    wasn't blown??? Mostly I've been just trying to get product out the
    door.

    Thanks for everyone's ideas... (I'm still scratching my head.)

    George H.
     
  17. No very hot and humid here, (70's), till today... (freeze warning for
    tonight)

    George H.
     
  18. Well there are a bunch of different circuits sharing the same power
    supply. I might be able to tell if there's a lot of current draw.
    But maybe not worth it. I'll just test the chaos bit first.

    George H.
     
  19. halong

    halong Guest


    opam blown b 4 installation ? could it be counterfeit ?
    how many suppliers you hvae on this part?

    so, blame the supplier
     
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