Hi I would like to know if there is a practical way to measure micro currents for arduino. Luis
Not without external hardware, but for clarity, define "micro currents".
The measuring currents of the order x10^-6, I mean low currents
I use the uCurrent, but it seems to be out of production right now. You will need something similar.
Can I use an operational amplifier? what do you recomend me?
To measure microamperes you generally need a sense resistor and an amplifier. Here is an overview: http://circuitcellar.com/cc-blog/tips-for-measuring-small-currents/
luisemiliani02: Can I use an operational amplifier? what do you recommend me?
If you look at the page in jremingon's first link, then there is a link to the circuit diagram, and the other design files for that product. That should give you an insight into what you need to do.
luisemiliani02: Can I use an operational amplifier? what do you recomend me?
Arduino Leonardo has ADC with differential input gain of x200.
There is a post at Dorkbot on measuring uA with an instrumentation amp that might be helpful: