uCurrent opamp input resistor

[LionFishNL]

Hi all,

I was studying the schematic for the latest uCurrent:
http://www.eevblog.com/files/uCurrentRev5schematic.pdf

I understand all except the meaning of R12 which is placed at the positive feedback connection of the first 10x op-amp.
After some investigation I can't seem to come up for a reason for using a resistor there.
I am also interested on why that specific value was chosen.

Thanks in advance

[MLXXXp]

R12 provides a bit of input protection for the op-amp. The value is kind of arbitrary. It has to be low enough not to cause significant errors or frequency limitation but high enough to afford some protection. The value was probably chosen to be the same as R4 (as were R8 and R10, which protect the op-amp outputs) to minimise the number different part values required.

[LionFishNL]

Thank you for replying.
I can get the protection because of R8, it prevents from shorting on the output.
Probably for the same reason R10 is used.

But does the op-amp does not have a high input impedance making this resistor not unnecessary?

[MK14]

Thank you for replying.
I can get the protection because of R8, it prevents from shorting on the output.
Probably for the same reason R10 is used.

But does the op-amp does not have a high input impedance making this resistor not unnecessary?

The op-amp will probably have ESD protection clamp diodes, which will start to conduct if the input exceeds the supply rails, by about a diode drop or more. This could damage via excessive current the op-amp, if there was no resistor.

[MLXXXp]

The op-amp will probably have ESD protection clamp diodes

Thanks for explaining that. I should have pointed it out in my response.
For more info on protection diodes see:
EEVblog #831 - Power A Micro With No Power Pin!

[LionFishNL]

Thank you for providing the video.
It made me understanding the use of protection diodes and why that resistor is required.
Thanks to both of you

[Jannehrl]

Your lucky, i accidentally came to your post. I did ask myself the same question. 
I hope it helps. PS: Maybe you should`ve searched for it first

https://www.eevblog.com/forum/projects/fear-resistors-in-microcurrent/msg879017/#msg879017

have a nice day

[LvW]

I understand all except the meaning of R12 which is placed at the positive feedback connection of the first 10x op-amp.

I don`t think that R12 serves any "protection" purposes.
Instead, it is the common method for providing a kind of "current compensation": 
The DC input current (through the feedback path) into the inverting opamp terminal causes a small DC voltage.
This offset voltage will cause a corresponding offset at the output.
This could be a problem because the second Opamp is DC coupled to the out put of the 1st unit.
That is the reason we are using an additional resistor also in the DC input path for the non-inverting input - thereby producing a dc voltage of app. the mentioned offset voltage at the inv. input.
As the result, the difference of both dc voltage drops is very small and the corresponding output offset is reduced to a much smaller value.   

EDIT: In the meantime I have convinced myself that R12 serves protection purposes as outlined by other contributors. Thank you.

[Gyro]

Dave explained his reason towards the end of the thread that Jannehrl linked. He used it as a protection resistor and the value was chosen because he already had one elsewhere on the board.

To work as a DC bias compensation resistor, it would need to be the same value as the network on the inverting input - around 10k iirc.

Personally I think the resistor value is much too low to provide effective protection and would change it (I might actually pick 10k) if I owned one. That's just my opinion though.

P.S. There's another thread on uCurrent mods: https://www.eevblog.com/forum/projects/my-ucurrent-gold-mods/msg497595/#msg497595  maybe others too.