voltage to bidirectional current converter

[DanKalc]

Hi,
How to build a constant current generator from (-1mA to 1mA) controled by voltage. Google gives me links only to one direction current converters mainly 0-20mA. Is there a way to decaple DC from a current? I need my current source to measure impedance using 4 leads, so the current dont have to flow to GND but from lead1 to lead2.
Anyone got any idea how to build something like that?

[SeanB]

Use a differential input amplifier to do the sensing, that way you just use a current source to the ground of the supply. That is very common.

[Andy Watson]

Look up Howland Current Source. You will need a pair if you want to avoid ground - they need to be matched and perfectly balanced.