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DAC failure mode

Discussion in 'Electronic Design' started by Robert Latest, Feb 28, 2007.

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  1. Hi folks,

    currently I'm looking at a piece of equipment (a PC32 DSP board from
    Innovative Integrations) that has an AD669 D/A converter on board. Instead
    of the desired sweep from -10V to +10V, I get an odd assortment of linear
    pieces that looks like this:

    output
    ^
    +10 | / /
    | / /
    | / /
    +5 |/ /
    |
    |
    |
    0---------------------> command (0-FFFF)
    / /
    / /
    / /
    -5 / /

    Or, in words: When the output should be -10V (input 0000), it is -5V and
    rises linearly. On the 3FFF->4000 transition, the output jumps back to -5V
    and then rises properly to 0. On 7FFF->8000, I get a jump to +5V and a ramp
    to +10V, at BFFF->C000 back to +5V, and another ramp to +10V which is
    reached at FFFF.

    Question: How can such an error arise? Is this a typical DAC failure mode?
    Is it possible to cause it through ESD at one (buffered) analog output?
    Stupid question of course. Anything can be caused by ESD. I wouldn't have
    expected a commercial product to be that susceptible though.

    The background is as follows: I've been asked by a colleague to look into a
    problem that he seemed to have with some circuit connected to the DAC output
    in question. To test that other thing (which turned out to be healthy) I
    connected it to the PC32's output, found weird behavior, and then found the
    problem on the PC32 itself. Now I'm being accused of having destroyed the
    PC32 by touching its outputs. I won't go into the details of the
    accompanying issues here, but up to this point I was the only one who didn't
    have severe problems with this socially weird fellow, who is now convinced
    that I have destroyed his card and expects me to repair it. Personally I
    suspect that the card had been shot before and he'd just not noticed it in
    his measurements (which isn't all too plausible either). Malevolence on his
    part, weirdness aside, can be excluded.

    robert
     
  2. From the looks of it, I'd say the bit with weight 4000 is stuck on. If
    that isn't visible at the DAC input bus, then we must conclude that
    something inside the chip is shot.

    Regards,
    Jeroen Belleman
     
  3. It looks like the dac's 2nd to the MSB bit is stuck high
    all the time, perhaps a problem on the PCB? At any rate,
    that's an issue on the input side of the dac, not on the
    output side, so "touching" the card couldn't be a cause.
     
  4. Yeah, looks like it. Now I've got to find an ISA raiser card to look into
    that problem, and get my head ripped off again if caught doing it.

    robert
     
  5. John  Larkin

    John Larkin Guest

    Inspect the soldering. Most problems like this turn out to be visible.

    John
     
  6. In fact I did, and it looked promising: The DAC712 (not the AD669, contrary
    to the board's datasheet), had obviously been reworked in the past. It was
    hand-soldered and came from a different batch than the other two. Its D14
    pin (being in a corner of the package) was easily inspectable under a
    microscope and looked suspicious, and indeed, I could get a scalpel into
    where there should have been solid solder. Rather than taking the chance of
    a pad that might have been damaged during the rework, I directly soldered a
    wire to the D14 pin of the neighboring DAC712 (both on the same data bus).

    Seldom had I been more confident of having found the right fix, so I was
    quite disappointed when the problem remained (intermittent, by the way, as
    I've found out in the meantime. Sometimes the darn thing works).

    Lacking proper logic analysis equipment, I clipped a scope to my wire and
    played with the DAC's inputs (unfortunately I can't set them directly but
    have to mouse-move a slider on the application that talks to the board). The
    D14 line sits frozen at 0V, although it should of course cycle high and low
    at times when it should be set high for this particular DAC.

    Then I started the part of the application that ramps the other DACs (all of
    which work flawlessly). Once this is running, the D14 line starts hopping
    merrily between 2.5 and 5V (!!!). When I stop the scanning process, the D14
    line sits back at ground. Capacitive coupling through a faulty trace? Why
    the three-level thingy, then? Could it be a measurement thing -- because I
    don't have an ISA riser board, I couldn't fit a proper scope probe onto the
    board but had to use clips and banana-plug cables.

    More headache. I'm actually not hell-bent on fixing this; having it repaired
    externally is no problem. The problem is the time factor involved in
    shipping this thing back and forth overseas, and my piece of wire would have
    been just so beautiful.

    robert
     
  7. Fred Bartoli

    Fred Bartoli Guest

    Robert Latest a écrit :
    Not capacitive. Just a dead short between 2 outputs, and the drivers'
    RDSon divide according to their status.

    Go hunting for another data/signal line showing the same pattern and
    you're almost done.
     
  8. Joerg

    Joerg Guest

    Or (but more unlikely) something going into tri-state.

    Or trace it back to the respective bus driver chip and see if that looks
    healthy. Quite possibly a solder joint has come off at that chip.
     
  9. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :
    Tssttt... With a sig line hoping between 2.5V and 5V?
    Not impossible. But very unlikely.
     
  10. Joerg

    Joerg Guest

    It depends on what else is hanging on that line. For example, you can
    have that very scenario with a thevenin termination. High -> 5V or
    whatever VCC is, tri-state -> 2.5V or whatever the thevenin ratio has
    been set to.
     
  11. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :
    OK, you almost win. Almost, because it's an ISA card and I don't see
    Thevenin terms used there :)
     
  12. Joerg

    Joerg Guest

    Well, sans Thevenin you theory about two outputs fighting each other
    looks more likely. That should be quite easily detectable via touching
    the chips. When it goes "Ouch!", then that's one of 'em ;-)
     
  13. Lionel

    Lionel Guest

    [I'm assuming that your DAC is a high-density package, with a zillion
    pins on 4 sides.]

    The first thing I'd look for is a short (likely a solder whisker, from
    your description) between the D14 pin & the next pin over. If so, you
    should be able to detect it (powered down, for preference) with a
    multimeter.
    It's a very typical kind of fault on boards with high density chip
    packages (eg; QIPs, etc), especially ones that've ben banged around by
    service staff, because they pins are so close that a tiny bump can
    bend them just enough to touch. I fix those by sliding the point of a
    scalpel between the pins to separate them.

    Another common one is an invisible dry joint *under* the 'L' part of
    the pin, which typicially shows up as an intermittant fault that gets
    progressively worse over time, or with heat or vibration. The scalpel
    trick will often reveal that problem as well.

    The easy repair is to lift the pin with a blade or fine needle,
    solder-wick & tin the pad, do the same for the pin, then re-tin the
    pin, bend it down, & dab it onto the pad with a *freshly cleaned* fine
    iron. Test for shorts between the pin & the ones on either side, & you
    should be all fixed.
     
  14. Lionel

    Lionel Guest

    The 2.5V will be because it's shorted to another line, & you're
    getting 2.5V when one is trying to go high, but the other is trying to
    go low. Betcha the chip driving the short are both running hot. ;)
     
  15. Right. Sounds reasonable. At the moment I don't have access to the card so
    I need to speculate a bit (an interestingly nested problem here):

    1. If this short exists, shouldn't all four DACs (or even more parts; I
    don't know how many share this particular segment of the data bus) show
    weird behavior?

    2. The 712's logic threshold is somewhere between 0.8 and 2 volts. Yet it
    only works properly while I see all the 2.5-5V stuff going on on the D14
    line. When D14 is solid low, the 712 behaves as if it were stuck on high. Of
    course a lot could be clarified here if I could just have a quiet sit-down
    at the bench, triggering on CS and whatnot. Suffice it to say that this is
    not possible at the moment.

    3. The fact that this particular 712 had been replaced before (by whom I
    don't know -- the card has a somewhat muddy history) suggests that this
    problem had been there earlier, and that it was erroneously traced to the
    712 when in fact it was shorted outputs or some other cause.

    4. All this is moot because the card will be sent out for repair for two
    reasons:
    a) I'm currently being blamed for having caused the defect (which
    is ludicrous because I don't have telekinetic capacities). If I were to
    try my own hands at a repair, one of the following three things will
    happen:
    - I manage to fix it but I won't gain anything because in the eyes of the
    beholder I just righted my own wrong as I damn well should.
    - If I can't fix it it will still have to be sent out.
    - If I break the thing altogether I'll take even more blame.
    In short: I can't win but can only lose.
    b) I neither have the schematics to this (6-layer) board, nor the proper
    measurement outfit nor SMD rework equipment.

    But thanks for bearing with me. The shorted-output idea nicely fits the D14
    behaviour on the scope, but not the proper operation of the other devices on
    the bus.

    robert
     
  16. This signal isn't on the ISA bus but on the data bus of the TMS320 on the
    board. Or at least the part of the bus that feeds the DACs; there may be
    drivers in between (I don't have any schematics).

    robert
     
  17. It's a big 28-pin SOIC (1.27mm lead spacing)
    That's exactly the way in which the D14 pin of that chip looked suspicious.
    I fixed it (with a proper piece of wire, not just reflowing that pin) but to
    no avail).
    Yeah, it seemed to. I mentioned it in an earlier post.

    robert
     
  18. Lionel

    Lionel Guest

    Jumpering it is not that good a way to fix that kind of fault. You'll
    be more likely to get a reliable repair via the method I described (or
    any other method that ensures that the pin is soldered back where it
    should be).
    Seriously, ditch the jumper & redo the pin properly. You might be
    surprised what you find under it when you lift the pin under a
    magnifier & a really bright light.
     
  19. Lionel

    Lionel Guest

    Oh boy, I can really relate to your situation. I've recently parted
    company with an obnoxious & rabidly clueless client who repeatedly
    blamed his network (that I built) for faults that he'd caused himself.
    His most recent self+foot+gun disaster was when he crashed the servers
    & switches by scheduling builders to perform structural work in his
    office - in the middle of the working day - without informing me, or
    taking any precautions, or at least shutting down all the electronics.
    He then put the cherry on top by letting the builders plug their saws,
    grinders & **ARC WELDER** into the same power circuit supplying the
    server, etc. Needless to say, the moron blamed the failure on me.

    Sorry Robert, I hadn't seen this part of the thread when I wrote my
    posts. Obviously, it makes more sense for you to wash your hands of
    the whole thing. ;)
     
  20. Never mind. I learned a great deal in this thread. Thanks, everybody, for
    contributing.

    Now I have a little electronics problem that's more domestic. See another
    thread.

    robert
     
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