4-20 ma analog output. circuit for increased resolution
You don't say what resolution your A/D is, but you probably don't have a problem to begin with.  A 12 bit A/D (quite common and available built into a number of microcontrollers) has a resolution of 1/4095 of its whole input range.  If that is 3.3 V, then the resolution is 810 &micro;V.

You want to detect a 200 &micro;A difference thru a 120 &Omega; resistor, which amounts to 24 mV.  That's 30 times the resolution of the 12 bit A/D.  Put another way, you will see a 30 count difference in two readings that are 200 &micro;A apart.  Even a 10 bit A/D would give you about 7.5 counts.

Where is the problem?