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

You may have to go with a rail-to-rail type instrumentation amp.

Hi, thanks for the suggestion! I thought the AD623 was already a rail-to-rail instrumentation amp. Isn’t it?

kemaltekbas said:

Hi, thanks for the suggestion! I thought the AD623 was already a rail-to-rail instrumentation amp. Isn’t it?

Click to expand...

How near do you need it to be?, rail-to-rail isn't actually real - it's just 'nearish rail to nearish rail'.

In my project, I’m designing a pressure transmitter, and I need the 0-50mV differential sensor output to map precisely to a 4-20mA current output for PLC. This is critical because any deviation could result in a loss of pressure measurement accuracy, which would affect the system’s performance. I understand that rail-to-rail isn’t exactly 0V but rather "nearish" to the rails, as you mentioned. However, I’m concerned that even a small offset might cause significant errors in my application. Is there an alternative way to design this circuit to ensure the 0-50mV input maps exactly to 4-20mA, without losing precision? I’d really appreciate any suggestions!

kemaltekbas said:

Hi, thanks for the suggestion! I thought the AD623 was already a rail-to-rail instrumentation amp. Isn’t it?

Click to expand...

My mistake.
Yes it is.

Something to try:
Add a diode (e.g. 1N4148) from the Ref pin to the negative rail to raise the IRET/U1Ref voltage for Ref and the op amp (-) input about 0.7V above the negative rail.
Add a 10KΩ resistor from the positive rail to the Ref voltage to provide bias current.

I use the XTR115, and the IRET is 0v referenced to the 2.5v output I run a precision op amp at 2.5 v.

I dont think your XTR116 is quite right.. the output stage on a real device has a darlington set up.

Without a "true" XTR116 part, your pretty much guessing.

Ian Rogers said:

I use the XTR115, and the IRET is 0v referenced to the 2.5v output I run a precision op amp at 2.5 v.

I dont think your XTR116 is quite right.. the output stage on a real device has a darlington set up.

Without a "true" XTR116 part, your pretty much guessing.

Click to expand...

To improve my simulation and better understand the circuit’s logic before testing it in real life, is there a way to find a true SPICE model for the XTR116? Alternatively, do you have any suggestions on how I can simulate the XTR116 more accurately, especially considering the Darlington output stage? I’d really like to ensure my simulation aligns with the real device’s behavior as much as possible.
Thanks again for your help!

Spice models :

I couldn’t find the SPICE model under the Design Tools and Simulation section on these pages. Am I looking in the wrong place?

If you download those tools the models are in their libraries. However I looked in Cadence tool,
so far no luck.

I found this - https://e2e.ti.com/support/amplifie...ing-as-espected-from-library-of-tina-software

I downloaded it, opened with wordpad, and I think its encrypted. Suggest you post on TI
forums to see if you can get a standard PSPICE file or even the Cadence file.

I’ve already checked both programs but couldn’t find the component.

Got this from ChatGPT :

Texas Instruments does not offer an official SPICE model for the XTR116 4–20 mA current loop transmitter. However, a functional behavioral model is available within the TINA-TI simulation software. This model is designed to emulate the device's general behavior rather than provide a detailed transistor-level simulation.

Accessing the TINA-TI Model​

You can find the XTR116 example circuit in the TINA-TI software under the "4–20 mA Loop Analyzer" example. Please note that this example contains a known issue where the amplifier is incorrectly oriented. To correct this:

1. Right-click on the amplifier component in the schematic.
2. Select "Mirror" to flip the component.
3. Rotate the amplifier to the correct orientation.
4. Paste it back into the circuit.

After making these adjustments, the simulation should more accurately reflect the expected behavior of the XTR116. citeturn0search0

Alternative Simulation Options​

If you're seeking a more detailed simulation, you might consider using PSpice for TI, which offers a comprehensive simulation environment with an extensive model library. While the XTR116 may not have a dedicated model, you can explore similar devices or create a custom behavioral model to suit your simulation needs. citeturn0search4


If you need assistance with setting up the simulation or have further questions about modeling the XTR116, feel free to ask!