# Over-voltage protection for op amp

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

18
1
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

Any sugestions?

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2. ### Alec_t

2,913
791
Jul 7, 2015
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.

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. ### Audioguru

3,043
678
Sep 24, 2016
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. ### Audioguru

3,043
678
Sep 24, 2016
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.

6. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
651
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. ### Audioguru

3,043
678
Sep 24, 2016
A diode will clamp the input voltage to be higher than the maximum allowed input voltage.

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,452
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Jan 21, 2010
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. ### Audioguru

3,043
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Sep 24, 2016
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. ### Alec_t

2,913
791
Jul 7, 2015
Belt and braces. I wouldn't like to rely on a 'probable' diode .

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,452
2,812
Jan 21, 2010
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.