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Looking for opamps with ability to tolerate input voltage above V+ ?

Discussion in 'General Electronics' started by Christopher Ott, Dec 12, 2005.

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  1. I have a board which is currently using several LT1491's, specifically for
    their ability to tolerate up to 44 volts above V+ at the inputs. The down
    side, is that they are expensive, and frequently out of stock. I'm curious
    is anyone out there is familiar with a similar device which might be a bit
    cheaper. I'm using this on heavy equipment with a 24v system. Commonly the
    LT1491 has a V+ of 5v, however the inputs could momentarily be as high as
    28v (due to miswires in electrical assembly).

    Any ideas?

    Thanks,

    Chris
     
  2. Tilmann Reh

    Tilmann Reh Guest

    (setting a Fup2 s.e.c)

    If the high input voltages are only due to miswires, you might as well
    add protection circuitry to a more common, standard OP.

    When looking for OPs that accept voltages significantly above V+, they
    all are rather esoteric, expensive and/or hard to get.
    The good ol' TL072, however, at least accepts input voltages /up to/ V+,
    which might be sufficient in some cases.
     
  3. I do have a 30v TVS (400Watt SMA package) protecting the LT1491 inputs from
    ESD and short duration transients such as alternator noise. So I'm really
    just trying to protect the inputs from 5 volts to the point where the TVS
    turns on (around 33 volts.) Miswires in electrical assembly are
    unfortunately frequent enough that I have to accommodate for them. They will
    always be +24v to +28v depending on if the engine is running. The LT1491
    works very well for this, since it's inputs are so robust, but at $4.25
    each, I feel like I'm getting raped.

    Any elegant ideas to protect a normal opamp while keeping the parts count
    low?
     
  4. Richard

    Richard Guest

    Zener diodes??
     
  5. default

    default Guest

    Add a clamping diode from the input to the positive supply rail of the
    op amp, and add a small resistor on the input signal to keep it from
    frying the diode (assuming there isn't already one there for the
    circuitry)

    That sort of problem could be designed out.
     
  6. Jasen Betts

    Jasen Betts Guest

    ["Followup-To:" header set to sci.electronics.]
    the way I'm reading that, they just need to survive, they don't need to
    function correctly in those conditions - right???
    two:

    build your own op-amps from discrete transistors.
    pass labs have an article on their web site.

    use series resistors (eg 1K) and shunt diodes to VCC or to a 3.9V zener,
     
  7. CWatters

    CWatters Guest

    in message
    and might eliminate need for a fancy ESD protection device as well.
     
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