Precision Voltage-to-Current Converter Circuit

The AD620, along with another op-amp and two resistors, makes
a precision current source circuit.

The op-amp buffers the reference terminal to maintain good CMR.
The output voltage, VX, of the AD620 appears across R1, which
converts it to a current. This current, less only the input bias current
of the op amp, then flows out to the load.
where
Vx = [(Vin+) – (Vin-)] * G
IL = Vx / R1
Circuit operates on 1.8 mA, ±3 V

Gain(G) is resistor-programmed by Rg
Rg = 49.4K ohm / (G-1)