@mzzj - Thanks for the thought about the 4-20mA loop transmitters, and I see the PR Electronics 4114 / 4501 are actually quite cheap.
@texaspyro - The Valhalla 2555A Transconductance amp is fantastic, 100A, nice. Although I can see it is meant to handle low currents too, it may be a but too brutish for our LED driving
I can't that their more gentle calibrators have transconductance - maybe I missed something.
@Jay_Diddy_B- thanks for that alternative design, v nice.
We're coming round to the idea that we'll have to build - I'll keep you posted for anyone else looking at this in future.
Alan