I found this description that may help.
"Voltage To Current Converter Circuit , Lakehead University, Chemical Engineering
The Voltage to Current converter circuit is designed to interface the 0 – 5Volt output of the
Matheson Model 8143 Mass Flow meter to the 4–20mA current loop input of the Bailey Freelance DCS.
The circuit consists primarily of the Burr – Brown XTR110 Voltage to Current converter chip. This chip
can take multiple ranges of voltage input and will give a linear and proportional current output of various
ranges. The configuration of the chip determines the input and output ranges. The following schematic
shows the connections required to have a 0–5V input and give the proportional 4–20mA output current.
The output from the XTR110 are source and gate voltages for a P-channel MOSFET or can be substituted
with the capacitor C2 and the PNP 2N3906 bipolar transistor. The chip allows for callibration via the zero
and span adjust potentiometers R1 and R2.
The remainder of the circuit is a 24Vdc power supply required to drive the XTR110.
http://electricallabs.lakeheadu.ca/projects/others/projects/v2c.pdf