Help with 4-20mA System XTR116: AD623 Offset Issue at 0mV Input

[kemaltekbas]

Hi all,
I’m new to circuit design and working on a 4-20mA 2-wire loop-powered system to convert a 0-50mV sensor output using the XTR116 (simulated based on the datasheet since I couldn’t find a library). I’m stuck on a couple of issues and need help:

1. My sensor has a common mode voltage (50% of input). I added a voltage source at the AD623 input to simulate this. For 1-50mV inputs, I get 4.32-20mA output, which is great. But the AD623 output has an offset from the REF pin (tied to my simulated IRET). Oddly, the current through my 25kΩ resistors is always correct (e.g., 160µA at 50mV). Why is this happening?

2. At 0mV input, the AD623 output doesn’t go to 0V due to the REF pin offset, so I get 7mA instead of 4mA. How can I fix this to ensure 0mV gives 4mA?

Thanks for any help!

 

[KlausST]

My sensor has a common mode voltage (50% of input)

What sensor? --> link to datasheet?
Your sensor has an electrical input? ... Or do you mean the "power supply"?

I added a voltage source at the AD623 input to simulate this.

What is this "voltage source"? If your answer is "V2", then I guess this is the wrong way. But surely depends on the used sensor.

But the AD623 output has an offset from the REF pin (tied to my simulated IRET).

"Offset" with reference to what? Usually - when not mentioned otherwise - this is GND. But the node you marked GND is not suitable as reference because it is "isolated" by R and D to your amplifier circuit.
--> If you want the measurement referred to the bottom of V2 (mind the missing junction dot), then put the GND symbol there.

2. At 0mV input, the AD623 output doesn’t go to 0V due to the REF pin offset, so I get 7mA instead of 4mA.

These given values make no sense at all.
I don´t believe that with 0.001V input you get 4.32mA ... but at 0.000V input it "magically" jumps to 7mA
No simulation pictutre proves this. --> show this simulation.

But ... you can´t expect any amplifier / OPAMP to output a lower or exactly the voltage of the lower supply rail.
Read the datasheet about "output specification".

Klaus

 

[kemaltekbas]

Hey,


Apologies for the lack of clarity in my previous message.


I'm using a pressure sensor Pressure Sensor(MPM289), and yes, when I mentioned V2, I was referring to the common-mode voltage.


I'm relatively new to this field, so I might be misunderstanding some terms or making some basic electrical mistakes.


What I was trying to explain is that when I attempt to simulate the behavior of my pressure sensor (which might not be entirely accurate, but it's the only approach that has yielded meaningful results so far), and I apply a differential input of 0–50 mV, I observe an output current of 4.32–20 mA at the Iout of the XTR116.


In my first test case, an input of 1 mV results in an output of 4.32 mA.
Theoretically, this makes sense: I'm using a gain (G) of 80 with the AD623, so a 1 mV differential input becomes 80 mV at the output. With a 25 kΩ resistor, this results in 3.2 µA. Then, using a 4.096 V reference and a 102.4 kΩ resistor, I inject an additional 40 µA into the internal op-amp of the XTR116. With the internal resistor network, this produces a 4.32 mA output, which seems to follow the correct logic for differential inputs between 1–50 mV.


I double-checked my setup: when I apply 0 mV as a differential input (i.e., both inputs tied to ground if it is a wrong way please share with me), I get 0 mA output. However, if I remove the 2V common-mode voltage, I get a 7 mA output. Why does this happen? How can ı get 4mA when I apply 0mV.

 

[FvM]

The relation of your simulation circuit to original pressure sensor circuit isn't quite clear. Therefore I don't understand why you have chosen your way of generating 4 mA current offset. Anyway it's clear that AD623 output can't swing down to -Vs. Connecting REF input to -Vs will always involve incorrect zero reading. Why don't you use REF input to set 4 mA reference instead, gives AD623 output in useful range.

Shown Proteus simulation measurements are almost useless to understand where measurement deviation is introduces, I would need voltage levels refrenced to AD623 -VS (Common node of left circuit part). There should be no problem to place GND symbol here instead at current loop -24V.

I presume the original transucer circuit is different in so far that it has no internal voltage sources, everything must be derived from loop input.