Current Sense Op Amp circuit not working as intended

Hi all,

I am trying to measure micro currents but the op amp seems to saturate right away or I’ve got something else going on - see attached schematic

I’ve created a current sense circuit using a 10 ohm current sense resistor and a ADA4528 precision op amp. The current I am measuring ranges from 0-30ua. Based on ohms law, a full scale reading would have .3mV across the 10 ohm shunt. Unfortunately I cannot use a higher resistance resistor due to it interfering in the circuit I am measuring. I have set the op amp to a 10k gain using 10k and 10 ohm resistors. Based on this a full-scale reading should give me 3v.

While operating the circuit the amp seems to saturate right away giving near rail voltage. Does the shunt have too low of impedance? Any ideas much appreciated.

What voltage is Source ? Very close to one of the power rails for the op amp ?

Maybe look here also: https://www.susumu.co.jp/germany/tech/know_how_09.php (9.1)

or better, here: https://www.digikey.be/nl/articles/fundamentals-of-current-measurement-part-2-current-sense-amplifiers

You are trying to have a single opamp stage with a voltage gain of 10000.

This is crazy. 100 is pushing it for a single opamp gain stage - things like gain-bandwidth product and input offset will start to cause issues as the gain increases.

You also have used the wrong topology. For a shunt you need a differential amplifier circuit: https://www.electronics-tutorials.ws/opamp/opamp_5.html

Note that both terminals of the shunt resistor need to be within the input voltage range of the opamp, so it can't be floating w.r.t. opamp supply, not outside its range. (assuming high gain circuit).

Thanks for the replies, i was under the impression that this was a differential amplifier but now I realize it’s not with the lacking resistors on the non-inverting input. Will go back to the drawing board

http://eevblog.com/files/uCurrentArticle.pdf

You could use the circuit of my millivoltmeter with your shunt. as its a differential amplifier its resistant to source voltage (within limits) and will give you the gain you need. http://www.skillbank.co.uk/arduino/measure3.htm

It would behoove you to use standard schematic symbols rather than IC packages in your drawings. After a while your brain grows a part that "thinks like an op amp" and can grasp the circuit operation just by looking. That doesn't happen with labeled boxes.

Thanks for all the input. I was able to redesign the circuit and it is now working as expected! Thank you again for all of your help.

Glad its working for you now.