How to determine op-amp gain with active feedback?
What you need to do is add a couple of resistors







simulate this circuit &ndash; Schematic created using CircuitLab

If the instrumentation amp has a gain G, then, since the current through R1 must equal the current through R2,

Vin/R1 = G iL Rs/R2, 

where iL is the load current.

Rearranging the terms gives

iL = Vin(R2 /R1 G Rs)

Note that, strictly speaking, an instrumentation amp is not required, since Rs is grounded, and a simple non-inverting op amp would do the job. In practice, an instrumentation amp would be a good idea, since tiny differences in ground resistance will have a noticeable effect due to the large gain of the amp.

Also note that this configuration will almost certainly oscillate like crazy. The phase shift caused by the instrumentation amp will need careful compensation.