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Caps on 741 supply voltages

Discussion in 'Electronic Design' started by [email protected], Jan 28, 2006.

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  1. Guest

    Hi there, simple question that must be in H&H, but I can't find it. I
    have a PCB with about 18 LM741 op-amps on it. I supply +/-15V to V+ and
    V- via traces on the board (~1mm width -- pretty big). Now, I've heard
    that you should always have a cap. to ground right at V+ and V- because
    the traces on the board carrying power have inductance, and you want to
    be able to supply current to the op-amp fast. Is this right? I pretty
    much swing the output of the op-amps from 0 to -10V at no faster than
    ~1kHz. If so, what is a good value? I assume uF-ish is fine.

    Thanks!

    Jesse Wodin
     
  2. Tim Wescott

    Tim Wescott Guest

    You want to prevent parasitic oscillations in the chips. I always drop
    a 100nF cap from each supply to ground at each chip, and I usually try
    to make sure there's a 10uF electrolytic somewhere on the board.
     
  3. Guest

    Excellent, thanks! What's the 10uF electrolytic for though? I've
    always been taught to stay away from electrolytics, though I guess
    that's for RF purposes only.

    jesse
     
  4. Tim Wescott

    Tim Wescott Guest

    The little caps decouple each chip at high frequencies, the big caps
    decouple the whole board at lower frequencies.
     
  5. Guest

    I've got a nasty suspicion that the big electrolytics have enough
    equivaent series resistance to damp any high-Q resonances you might get
    between the wiring inductances and the 100nF ceramic disks.
     
  6. Joseph2k

    Joseph2k Guest

    The 100nF caps handle 200Hz and up pretty well, the 10uF is for lower
    frequencies.
     
  7. Rich Grise

    Rich Grise Guest

    When I was growing up, [ ;-) ], these were called bypass capacitors. I'm
    anal about them - in this case, I'd put probably about a 1~10 uF aluminum
    from each V to ground - you _do_ have a good low-resistance ground plane,
    right?

    There may be people who disagree with me, but for audio and maybe even
    video/low band HF, and always, always, with digital, you can't
    overcapacitate.

    (something in me wants to put a smiley there, but the answer isn't
    supposed to be flip - just its form. ;-) )

    Cheers!
    Rich
     
  8. wrote...
    Might, indeed! Your answer is the correct one, Bill, because a sea
    of 0.1 caps certainly makes a nice resonant circuit, somewhere in
    the 5 to 20MHz region. A single crummy electrolytic is just what
    the doctor ordered to kill that resonance. Better be crummy tho,
    no low-esr tantalums need apply. Ahem, crummy, but not too crummy!
     
  9. mook Johnson

    mook Johnson Guest

    We ARE talking about 741s here. Adequate for mid-fi audio but not high
    performance by any stretch.


     
  10. Pooh Bear

    Pooh Bear Guest

    Yes you can !

    Lots of bulk capacitance to ground simply provides a path for any ripple. You
    end up with a dirty ground.

    Graham
     
  11. Pooh Bear

    Pooh Bear Guest

    What do you mean by 'handle 200Hz and up'. Is the power supply no good
    for that on its own ?

    I think you have a fundamental misunderstanding of their purpose.

    Graham
     
  12. Guest

    The original 741 suffered from "pop-corn" noise, so-called because that
    was what it sounded like in audio circuits. Back in 1970, one of my
    then colleagues was using LM308s in a high-quality audio frequency
    front end purely because the processing necessary to make the
    super-beta transistors eliminated the pop-corn noise.

    Modern 741's are supposed to be free from this vice, but the last time
    I replace a 741 in a circuit, back in 1988, we did seem to get rid of a
    lot of pop-corn noise, and the effect was quite noticeable - it was
    controlling the induction heaters on a GaAs crsytal puller, and they
    went from bang-bang operation to the sort of proportional control
    they'd been designed to provide. The operaters were grateful and the
    GaAs crystals seemed to have fewer defects. This was the machine that
    made 95% of the single-crystal GaAs produced in the West (as it was
    then).
     
  13. wrote...
    Hey, even 741s can suffer from ringing on their supply bus lines.
    They pass this right on to the next stage, where common input-stage
    BJT nonlinear RF rectification turns this into distortion.
    Bill, what did you replace the 741s with in 1988?
     
  14. Guest

    OP-01. I subsequently did a fair bit to the front end electronics - we
    weighed the crystal as it grew with an LVDT on a spring, and the LVDT
    monitoring circuit got somewhat fancier under my care - but that was
    the sinble most effective improvement.
     
  15. Pooh Bear

    Pooh Bear Guest

    That's an interesting case.

    1/f noise may actually have been more problematic than 'poporn' in your
    instance though.

    Graham
     
  16. Guest

    The 741 hasn't got any spec for either, and neither does the OP-01.
    IIRR the OP-27 and OP-37 were amongst the first that did specify !/f
    noise - the LT1001 certainly does (0.3uV ptp typical 0.6uV max for
    0.1Hz to 10Hz) and I'd have gone for something with a specification if
    1/f noise had been a real problem.

    The LVDT was excited at a few kHz, and I amplified the output as much
    as I could before demodulation, so the OP-01 was more of a buffer than
    an amplifier by the time I'd finished.

    I'd have loved to have used a proper AC-bridge on the LVDT output, but
    there wasn't enough space for much electronics directly above the LVDT,
    and there was a great deal of expensive mechanical engineering in the
    way of getting any more space.
     
  17. Pooh Bear

    Pooh Bear Guest

    You *do* have a fundamental misunderstanding.

    Graham
     
  18. Joseph2k

    Joseph2k Guest

    No, it is not, that many trace inches away. That is why they are adjacent
    to the parts and not clustered elsewhere.
     
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