OPA2333: High-resolution two-path PWM to 4- to 20-mA converter

Hello User,

The second op amp is a servo that makes the right side or R3 be 0V

So R3 current is V2/R3 right; that current flows though R12 down

Voltage at R12, R20, bottom is -V2/R3*R12

So R20 current down is -V2/R3*R12/R20

The sum of R12 and R20 down current is -loop current = -V2/R3 + -V2/R3*R12/R20

Factor out -V2/R3 and you get (-V2/R3) * (R12/R20) as -loop current ; invert to get +loop current

If you do not get that then the second op amp servo didn't do its job, make right side of R3 = 0V

Thanks.
Loop supply and the converter doesn't share a same ground. 

Unless we are talking about earth ground, anyone can declare any node to be ground, zero voltage. I picked that triangle thingy to be ground for me.

I can see all of the loop current has to flow though R2 R20. With my declared ground and faith in servo doing it's job, I was able to recreated the formula.