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horrible development

Discussion in 'Electronic Design' started by John Larkin, Feb 26, 2007.

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  1. John Larkin

    John Larkin Guest

    OK, we have a BIST (built-in self-test) bus that can have sine waves
    from a couple volts to about 100 volts p-p. So I did this into an
    analog mux:

    | |
    | |
    | |
    bus--->------+-------r1-----+------| mux |-->--opamp-->--adc
    | | | hc4051 |
    | r2 | |
    | | | |
    | gnd | |---- +5
    | | |
    | | |---- -5
    +-------r3-----+------| |
    | | |----gnd
    r4 | |
    | | |
    gnd ________

    where one divider is about 4:1, for low level signals, and the other
    is about 21:1 for the big stuff. ADC range is +-3.5, and I can take a
    lot of samples and average to get dc, and simultaneously average the
    abs value to get ac.

    What happens is that when I have a big signal, selecting the
    high-ratio divider, the output of the 4:1 divider blows through the
    esd diodes of the mux and sneaks its way into the output, so I get a
    lot more signal than the 21:1 attenuated level I want, and it's of
    course distorted as well.

    Bummer. One of the HC designers once assured me this wouldn't happen,
    but that was another vendor (Moto) and the parts we have here are
    Fairchild so I guess different processes can do this.


    So, I can kluge on a couple of 1N5711's as clamps, really ugly, or
    find another drop-in part that doesn't blow through.

    Any suggestions?

  2. Eeyore

    Eeyore Guest

    How fast are the signals ? I'd be inclined to actively buffer and clamp.

    The other simple possibility is to fit your own ESD diodes in parallel and use
    some shottkys or those very low Vf types.

  3. Phil Hobbs

    Phil Hobbs Guest

    I've run into that too. Some parts connect all their outputs together
    when you exceed the supplies on one input. :-(

    Could you raise the impedance of the voltage dividers a whole lot? Or
    replace one of the 4 resistors with a small cap and adjust the frequency
    of the test signal?


    Phil Hobbs
  4. Might be a easy solution, current limit the ESD protection. You got an OPamp
    so Zin can be quite high.
    See my edit in your circuit. R5

  5. John Larkin

    John Larkin Guest

    This (brain damaged) version of the module generates sinewaves from 0
    to 100 KHz, but I run the self-tests at 10 KHz. Adding an active
    buffer would be a frightful hack.

    I found a couple of tubes of MAX4051's in stock, but they do the same
    thing. How can people design analog mux'es that do this?

    The fix is to add schottky clamps, the 1N5711's, from the 4:1 divider
    output to the +-5 rails. It finally passes all the BIST routines. I
    can go home now.

    Too bad: this is rev A of this 8-channel DDS synthesizer board (pic
    posted to abse a while back) and so far it has a single jumper;
    somebody forgot to run the uP WRITE- signal to an FPGA. So now it's a
    jumper and two diodes. Given a board this badly kluged, I suppose
    there will have to be a rev B soon.

  6. John Larkin

    John Larkin Guest

    A couple of schottkies, clamping the low-ratio divider to +-5, seems
    to work. Not too ugly a kluge.

  7. Joerg

    Joerg Guest

    The BAT54 comes in series pairs, making it one part. If it has to be
    super small even in SOT-523:,AT,CT,ST(SOT-523).PDF
  8. John  Larkin

    John Larkin Guest

    See my discrete diode kluge in abse.

    Actually, a sot-23 dual series schottky might be nice. If I solder pin
    3 to the resistor junction, the ends run to +5 and -5 without crossing
    over. I'll give that a try and see if it looks better.

  9. Joerg

    Joerg Guest

    Nice solder job. Do I see a tin whisker south of TP57? Just kidding...

    The BAT54S comes in SOT-23:

    About $0.03 in qties which is why I like them. Digikey has 72,000 of
    them in stock, which is kind of normal, which is why I like them even
    more. They'll still be around when I put my teeth in a jar. Imagine, a
    component that lasts a whole career.
  10. ehsjr

    ehsjr Guest

    Well, it may be a kludge, but wasn't it fortunate
    you had room on the board? BTW C135 doesn't look like
    it's a happy camper ...

  11. Fred Bartoli

    Fred Bartoli Guest

    John Larkin a écrit :
    There are some switches designed to handle fault over voltage up to
    (from memory) 44V.
    I think they are marketed for automotive environments and ISTR that
    there are some cloned on the 405x family.
    The keyword is fault tolerant I think.

    Don't have much time to search, but maxim comes to mind and probably
    moto, oops onsemi.

    If you go clamping and with a new rev. then I'd drop the leaky
    schottkies, go with a BAV99, split the divider's top resistance and
    clamp there.
  12. On a sunny day (Sun, 25 Feb 2007 16:00:32 -0800) it happened John Larkin

    You are using it out of spec, inputs should always be within clipping.
    If it ever worked you were lucky!
    Cannot you add a MOSFET or other switch to clamp the too high input to ground?
  13. Lionel

    Lionel Guest

    That designers would read the spec-sheet? It's not like there are lots
    of chips that /don't/ dump over/under signal voltages to their common
  14. Jim Thompson

    Jim Thompson Guest

    4052, or similar, so you can short the output leg of the 4:1 divider
    to ground?

    Or flip the 4051 backward so you pick a tap on a single divider...
    (after all the pins are called I/O ;-)

    ...Jim Thompson
  15. Jim Thompson

    Jim Thompson Guest

    Ooops! Strike that second suggestion :-(

    ...Jim Thompson
  16. John  Larkin

    John Larkin Guest

    The spec sheet names a maximum esd diode clamp current. It doesn't
    mention that the chip becomes non-functional at about 1% of that

  17. John  Larkin

    John Larkin Guest

    I think it is in spec, or at least that the spec is woefully
    misleading. It's screwing up at about 1% of the epecified esd clamp

  18. John  Larkin

    John Larkin Guest

    Dang, you beat me to it.

    But you can do a tapped divider and use the mux to ground taps on a
    series-string divider, sort of like what you suggest.

    But why do these chips do this? How does the esd diode current find
    its way into the output? Incidentally, the effect is almost perfectly
    symmetric for high and low clmping.

  19. Eeyore

    Eeyore Guest

    We've discussed relaying on those diodes before a number of times.

    It's the first time I've heard of it being problem but I'd not be inclined to
    rely on them myself.

  20. Jim Thompson

    Jim Thompson Guest

    On Mon, 26 Feb 2007 07:19:19 -0800, John Larkin

    I could be pompous and remind everyone how many times I've said DON'T
    DO IT ;-)

    ...Jim Thompson
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