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

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?

• 5x0.97 is 4.85 so this is a non-ideal input range, and gain does not fix it. Commented Apr 10, 2020 at 23:21
• Do you have any programmable options in the receiver, so you can fix it with software scaling? Commented Apr 10, 2020 at 23:46
• @relayman357 I can change the PWM output.It is limited to 0V-3V3 (PIC32) but I need the 0.97-2.75 as this is the range where my sensor is active (im reading from ADC input).I don't know how I could scale this to something usable Commented Apr 10, 2020 at 23:50
• You need to look at the receiving end of the 4-20 ma. If you have control of that you may have some flexibility.
– user80875
Commented Apr 11, 2020 at 0:57
• @SpehroPefhany gain need not be with respect to ground Commented Apr 11, 2020 at 3:39

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

• I Tested the second one in LTSpice with a circuit that sims a pwm as the input(behavioral source with pulses from 0 to 2.75 with a filter after) but for some reason it only gets to 2.75V when I sim the pulses to go up to 4V.Do you know why that might be?Othewise this seems to work 100%.Thanks Commented Apr 11, 2020 at 19:41
• it could be due to the 22K input resistance of the second circuit. but I'm sure that I understand what you designed. Commented Apr 11, 2020 at 20:01
• perhaps a buffer amplifier before the terminal I have called "input" will help. - that's just another op-amp with -inoput connected to outout) Commented Apr 11, 2020 at 20:04
• The buffer solved it!My LP Filter had a 10k Resistor so it was producing quite the voltage drop with the input resistance.Using the OP07 from Spice I get a 19.5mA output at max and 4.6ma at min.Still not perfect but I'll try to change the Amplifier and see if it changes it up thanks for you help Jasen! Commented Apr 11, 2020 at 20:23
• Could someone explain the theory behind this circuit?I know this is based in a differential op amp as a current source but I can't seem to grasp how it works Commented Apr 12, 2020 at 0:16

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