# 4-20mA current loop design

I have been working on 4-20mA transmitter project and stuck with a problem. My design is based on CN0319 evaluation board but I changed some of it, to make it works with other part of the design.

Below is my design, I want it to transmit 4-20mA current when the input of op-amp is PWM 0-100% duty cycle. It uses power supply 24V and drop it to 5.7V to supply entired circuit.

My problem is when I try to drive 4-20mA output current with 0-500 ohms load resistor. At around 400-500 ohms load, the circuit sense output current as desired. but when load resistor is 0-400 ohms. the current goes a little bit higher than it should be. For example, if I want to drive 4 mA, my circuit can drive 4 mA at around 420-500 ohms. But at 0 ohms, it senses about 4.2mA and it slowly goes down to 4mA when I increase load resistor to 420 ohms.

To drive 4-20mA, input voltage at V- of op-amp must be 0-0.6V. So I measure V- and V+ compared to DGND, V- voltage is constant at any load resistance but V+ is goes lower when load resistance is 0-400 ohms. What am I missing if I want to make the current consistant at load 0-500 ohms.

I measured voltage across Rload using oscilloscope and start to see ripple on it when I decrease the Rload. the more Rload goes low the more ripple voltage goes high. So, I measured voltage at Pin3 and see the ripple. But did not see that on input voltage at pin2. Input voltage at Pin2 is from PWM and pass through the low pass filter to make it DC.

This is the voltage at Pin3 at Rload = 356 Ohms

This is the voltage at Pin3 at Rload = 445 Ohms

• Is your OPA244 rail to rail on the output? – winny Aug 1 '18 at 7:35
• @winny Thank you for checking out my question. I'm pretty newbie and my english is not good. so sorry about that. Anyway, I have checked its datasheet, it says output voltage can go from 0.1V to (Vs+) -0.75V (SPECIFICATIONS: VS = +2.6V to +36V). So, I think it is rail-to-rail. – BEWTY Rodbanyang Aug 1 '18 at 7:53
• What winny is asking, is if the voltage at Pin 6 of the Op-Amp is swinging between 0v and 5V properly. – MIL-SPEC Aug 1 '18 at 8:03
• 0.1 V is not rail to rail (but almost) and could explain why you are seeing problems at low impedance and thus low output voltage. Can you try with a R2R-advertised one? Could you tolerate a ballast resistor in series? – winny Aug 1 '18 at 8:04
• Have you checked the output with an oscilloscope? – Spehro Pefhany Aug 1 '18 at 8:16