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Hot opamp

Discussion in 'Electronic Design' started by Jeroen, Jul 16, 2004.

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  1. Jeroen

    Jeroen Guest

    Hi,

    I'm using a LT1678 opamp running from +/- 14V. Somehow it runs very hot, too
    hot to touch, though it seems to work fine. I checked the datasheet again
    and the supply voltage range is +/18V. How should I interpret this? +18
    and -18V or +9 and -9? It nows run from +14V and -14V.

    Jeroen
     
  2. Single supply up to 36V. Or +/- 18V. Perhaps you should check the
    load on the output(s).
     
  3. CFoley1064

    CFoley1064 Guest

    Subject: Hot opamp
    Hi, Jeroen. You're OK on the power supply -- it's good to 36V, or +/-18V. The
    LT1678 uses up less than 3.8 mA per channel, so it shouldn't run hot like your
    post says.

    Go back to the data sheet and get the pinouts on the opamp (it's standard 8-pin
    dual opamp). Look at your wiring, check for shorts and miswiring.
    Double-check for shorts between the ears. Then double-check your load, and
    make sure it doesn't cause excess current draw. Take particular care not to
    drive a capacitive load with this op amp. Use an isolating output resistor to
    avoid problems. See the data sheet.

    http://www.linear.com/pdf/16789f.pdf

    If you're still stumped, you might want to look at the possibility that high
    frequency oscillations are causing your overheating. Remember the
    gain-bandwidth product on this op amp is 20 MHz. You have to be careful with
    layout, and be sure to use good high frequency bypass caps. If you're using
    one of those plug-in protoboards, that can contribute to your problem. Use a
    scope if you have one. With a GBW of 20MHz, just about any scope will allow
    you to see the oscillations, if they're there. Use a 10X scope probe if you
    have one to minimize ckt. loading. If you do have HF oscillations, change
    things to see if it affects amplitude, frequency, &c. The op amp is almost
    certainly doing exactly what it's supposed to be doing for the circuit you've
    put it in. You just need to find what part of the rest of the circuit isn't
    what you want.

    Good luck
    Chris
     
  4. Jeroen

    Jeroen Guest

    I designed an one off PCB for this prototype which I received and assembled
    today. Had only an hour or so yet to test it.

    It's a multichannel signal conditioner for an ADC to bring -10-10V
    differential signals into the range of the ADC. I buffer the reference
    voltage from the ADC with a unity gain buffer, followed by an amplifier with
    a gain of 0.5. I put a 100nF cap directly at the output of the opamp, maybe
    I have to insert a resistor?
    This buffered voltage is then fed to each channel conditioning circuit. This
    opamp seems to run the most hot. The LT1678 in a channel conditioner gets
    quite warm, but can still be touched.

    The whole unit is run from two DC/DC converters followed by low noise LDOs.
    Still there is quite some HF noise present, maybe this is causing trouble
    too?

    Anyway there is some work left to do to perfect it. I have a Textronix
    TDS3xxx 300MHz scope at my disposal, so I can see enough.
     
  5. Tim Wescott

    Tim Wescott Guest

    Have you checked that output with the big cap on it? Usually that's a
    good way to make an opamp oscillate, and then hot.

    Put your scope on it and see.
     
  6. Ban

    Ban Guest

    Jeroen wrote:
    Use an isolating output resistor to avoid
    To avoid the oscillation you can do a couple of things:
    1.) decoupling caps directly at the supply pins to the ground plane, the
    negative one is usually the most important.
    2.) The before mentioned isolating resistor of 100-220 ohms between output
    and load.
    3.) The buffers are most prone to oscillation, you can insert a 1k
    resistor||1n from output to -in
    4.) divide the reference before the buffer and put that 100n there.
    5.) put a compensation network between +in and -in of the opamp (100R in
    series with 1n)
    6.) put the same on the output pin, not after the isolation resistor.
    7.) put a 100p across the feedback resistor of an inverting configuration.
     
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