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Input of unpowered OpAmp

Discussion in 'Electronic Design' started by panfilero, Dec 19, 2013.

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  1. panfilero

    panfilero Guest

    I had a voltage divider which was suppose to drop down a voltage to 3V connected to the input of an OpAmp. And it worked fine. But I noticed, when Ipowered off the OpAmp, that 3V dropped down to 1.7V, I wasn't expecting this. What do you think would cause this? My guess is some input protection circuitry, maybe a couple diode drops or something?
  2. hamilton

    hamilton Guest

    Is the Opamp a secret ?

    Yes, some OpAmps have protection diodes some do not not.

    Yours, who knows.

  3. Joerg

    Joerg Guest

    There can also be internal BE junctions that are able do this. 1.7V
    sound like two BE junctions plus some internal resistance. If a
    substrate diode had come on it would be more like 0.6V.
  4. Den torsdag den 19. december 2013 22.32.12 UTC+1 skrev Joerg:
    if the diode is to Vcc trying to drive the opamp, it would depend on load and driving resistance

  5. maybe a couple diode drops or something?

    Yes, things like that happen..

    Depending on the type of Op-AMp you use. Since you didn't post the
    PN, I'll assume you are working with bipolar type inputs. These
    become diodes in a sense and could leak back to the power rails when
    no power is present to counter act the circuit.

    Have you thought about using a bifet op-amp? I can't say it'll
    fix your problem however, it's worth a try.

  6. panfilero

    panfilero Guest

  7. Den mandag den 23. december 2013 16.26.11 UTC+1 skrev panfilero: INPUT OVERVOLTAGE PROTECTION
    Although the AD8628/AD8629/AD8630 are rail-to-rail input
    amplifiers, care should be taken to ensure that the potential
    difference between the inputs does not exceed the supply voltage.
    Under normal negative feedback operating conditions, the
    amplifier corrects its output to ensure that the two inputs are at
    the same voltage. However, if either input exceeds either supply
    rail by more than 0.3 V, large currents begin to flow through the
    ESD protection diodes in the amplifier.
    These diodes are connected between the inputs and each supply
    rail to protect the input transistors against an electrostatic discharge
    event, and they are normally reverse-biased. However, if the input
    voltage exceeds the supply voltage, these ESD diodes can become
    forward-biased. Without current limiting, excessive amounts
    of current could flow through these diodes, causing permanent
    damage to the device. If inputs are subject to overvoltage,
    appropriate series resistors should be inserted to limit the diode
    current to less than 5 mA maximum.

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