The below link is the site of Apex Microtech. They are a very professional outfit that makes very high power op-amps.
**broken link removed**
Click on App note #13: Voltage to current conversion. In that app note pay attention to the circuit called a "Howland Current Pump".
Note: If you build the grounded-load circuit in Figure 7, be aware that the circuit is very sensitive to resistor tolerance. I suggest 0.1% or better metal film for all Ri and Rf. This does not concern the very low resistance of Rs. Rs is (1ohm or so). Any resistance tolerance error of both Ri's and Rf's should be small when compared to the value of Rs.
As a matter of fact, all of Apex's app notes are interesting and well worth reading for any engineer or technologist.
Bob