TL081: Nano amp current measurment

Rakesh,

There are 892 different op amp models that could measure nano-amps of current.  Basically, any CMOS or JFET device will have picoamps of bias current, so they can measure nano-amps.  You should avoid bipolar as they will have nano-amps of bias current.  The TL081 device that you mentioned in your post has a bias current of 120pA max at room temperature and 5nA at high temperature.  If you are operating at high temperature, you should choose a special device that has ultra low bias current across temperature(e.g. OPA928, LMP7721.  You may also need to use guarding on the inputs.  The https://www.ti.com/tool/OPA928EVM uses guarding and the article series below covers the topic in detail.  It really depends on what minimum current you need to measure and at what temperature you will need to operate as to what device you should choose.  Of course, there are many other consideration such as supply range, bandwidth, slew rate, power consumption, and other parameters.  Regarding the gamma-radiation, will the device be exposed to the radiation?  If immunity to radiation is required you may need a space rated device as many devices will be sensitive to radiation.  

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-one/

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-2-component-selection/

https://www.edn.com/design-femtoampere-circuits-with-low-leakage-part-3-low-current-design-techniques/

Best regards, Art

Thank you Arthur