4ma-20ma loop from 0.97V to 2.75V

Question

I'm trying to develop a 4ma-20ma loop from an 0.97(4ma)-2.75(20ma) input.

This input comes from the output of a low pass filter of a PWM module. I've searched the web quite thoroughly but all the circuits I come across are the typical 1V-5V with the 250 ohm resistance after the r2r op amp.

Is there any way I can implement this without using a gain stage before?

As I can't implement this with a fixed resistor as I need my resistor to somehow change from 242-137 ohm.

UPDATE1:So i figured out how to scale down the output to 0.5-2.3 this way I can use a gain stage to atleast get a 1V-4.6V Output but this way my max will be at 18ma instead of 20.Any suggestions?

Answer 1

this will get you failry close.

^{simulate this circuit – Schematic created using CircuitLab}

but this is will be less effected by temperature variations

^{simulate this circuit}

Answer 2

This simple current sink will give you 4-20mA for 0.55 to 2.75V. Since you're only using the 0.97 to 2.75V you lose the bottom 20% of the range if you don't scale the PWM within the PIC to the 0.55 to 2.75V range. Typical accuracy is in the 0.1% range, depending mostly on the accuracy of R1. Output impedance is close to infinite within the compliance range of about +4 to +24V, so RL of up to about 1K with a 24V supply.

^{simulate this circuit – Schematic created using CircuitLab}