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Over-voltage protection for op amp

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Discussion in 'General Electronics Discussion' started by AdamZ, Mar 13, 2017.

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  1. Mar 13, 2017 #1
    AdamZ

    AdamZ

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    Mar 17, 2016
    Hi all,

    I would like to add an over-voltage protection resistor (Rp) for my AD549 op amp, which is occasionally subjected to 35 V at its inverting input. The op-amp is supplied +/- 5 V.

    The manual indicates that Rp should be chosen to limit the current to 100 uA.

    I am not quite sure how to choose the right resistor, because the op-amp itself has a significant input resistance.

    Today, I tried a 1 MOhm resistor (wanted ~300 kOhm instead), but this seemed to add a few standard deviations of noise, in addition to what seemed like a drifting voltage (or slow response) at the op-amp output.

    Figures:
    Op Amp over-voltage.png -my circuit without Rp
    AD549.png -from manual

    Any sugestions?
    Adam
     

    Attached Files:

  2. Mar 13, 2017 #2
    Alec_t

    Alec_t

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    You could connect your input via a resistor R1 to the anode of a diode having its cathode connected to the +ve supply. That will clamp the input to 0.6V above the supply. Diode current max=(35-5-0.6)/R1. A second resistor R2 could go between the anode and the inverting input. R2min=0.6V/100uA=60k.
     
    (*steve*) and AdamZ like this.
  3. Mar 13, 2017 #3
    AdamZ

    AdamZ

    18
    1
    Mar 17, 2016
    Good solution, which I will implement; however; I need an even simpler single-resistor solution for the next two weeks or so before the diode shipment arrives.
     
  4. Mar 13, 2017 #4
    Audioguru

    Audioguru

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    The input current of an AD549 is almost nothing. Your negative feedback resistor value is very high at 5M so an extremely high value protection resistor of 300M (not 300k) is needed which creates a lot of noise.
     
  5. Mar 13, 2017 #5
    Audioguru

    Audioguru

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    Sorry, the 300k resistor value is correct and will not add much noise compared to your very noisy 5M resistor.
    If an input voltage 0.6V higher than the supply is acceptable then you would think the datasheet would say so.
    Instead, the datasheet says the absolute maximum allowed input voltage is equal to the supply voltage.
     
    AdamZ likes this.
  6. Mar 13, 2017 #6
    CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Adam, if every request on EP could be resolved with a single resistor there wouldn't be much point in keeping EP up and running. There must be some scrap electronics you could cannibalize for a single common Diode?

    Chris
     
  7. Mar 14, 2017 #7
    Audioguru

    Audioguru

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    A diode will clamp the input voltage to be higher than the maximum allowed input voltage.
     
  8. Mar 14, 2017 #8
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    That's why you have the resistor as well (or even 2 resistors, one before the zener to limit zener current, and one after to limit input current).
     
  9. Mar 14, 2017 #9
    Audioguru

    Audioguru

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    The input has a maximum current rating because it probably already has a protection diode which has a low current rating. The protection resistor shown in the datasheet does not show an additional diode so why add one?
     
  10. Mar 14, 2017 #10
    Alec_t

    Alec_t

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    Belt and braces. I wouldn't like to rely on a 'probable' diode :) .
     
  11. Mar 15, 2017 #11
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Because it allows you to use a lower value resistor (for lower noise). For logic, it will also improve speed by preventing the high series resistance from creating a significant RC delay.
     
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