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XTR115: 4-20mA / 0-10V Output

Rodrigo Vernini
Intellectual 280 points
Part Number: XTR115
Other Parts Discussed in Thread: LM317, RCV420, XTR111

Hi,

i'm developing a sensor that should have 4-20mA and 0-10V output, jumper selectable.

The sensor has 3 connections, PWR, OUT and COM. 

For most applications it should work with 4mA, and could use 2 wire connection (PWR and OUT). It will have an option with displays and relays, that would consume more than 4mA, so it would have to insert COM line for extra current.

It could be powered with AC, and in this case should connect COM line.

Basically the connection are those:

Usually i would simulate on LTspice, but i haven't find the model for XTR115, and i'm very uncomfortable developing those analogical and not usual ground separation, so i need some help with this. 

I started with the circuit below.

GND - Basic functionality of the sensor, powered by 5V XT115.

COM-GND - Would return the current from optional extra functionalities, Display, relays, etc, powered by other sources (LM317 in example)

The jumper should pass 1A or 2A to A, same for B and C.

"A" should make the current pass trough the resistor, making voltage output.

"B" line should tell uC that it's Voltage selected, changing the range of the DAC-OUT1.

"C" - Nothing yet.

Some doubts:

Does R16 work for that desirable Voltage output?

Does the ground separations are correct?

Will the circuit work with the AC option?

  • 0
    TI__Mastermind 23050 points
    Hi Rodrigo,

    There are a few things to be concerned about here:

    1) Anything that is powered off the XTR regulator or reference must return its current to IRET, and this must still not exceed 4mA. The input circuitry (DAC) must also be referenced to IRET for proper functionality.

    2) You can not use a 2-wire device in a 3-wire configuration. If your input circuitry is referenced to your loop supply then you must provide isolation between this circuitry and the XTR input so that IRET can float relative to your common line. You may find this blog useful: e2e.ti.com/.../2-wire-4-20ma-sensor-transmitters-background-and-common-issues-part-5

    3) XTRs are not intended to be operated with ac supplies, if by 24Vac you mean +/-24V then this will damage the device. You will need to incorporate a rectifier to ensure that device is always powered with dc and has sufficient compliance headroom to source the voltage/current required.

    4) You can use a resistor as shown to do the voltage conversion. If you wanted something more precise you could look at the RCV420.

    Here are some additional resources that might help: e2e.ti.com/.../3533.xtr-resources
  • 0 in reply to Zak Kaye
    Intellectual 280 points

    Hi Zack,

    many thanks for the reply.

    I've been working in the circuit based on your text.

    1) DAC is on the same IRET (uC is IRET referenced)

    2)I have some digital data that i need to pass from the IRET ref. circuit to COM ref. circuit, as i have just 1mA left i found this one.

    3) There's already a simple rectifier in the circuit, diode and capacitors, i just wasn't sure it would work the way it need to comply with the wiring schematic above. 

    4) Unfortunally i have some budget target for the final circuit.

  • 0 in reply to Rodrigo Vernini
    TI__Mastermind 23050 points

    Hey Rodrigo,

    I see what you are after now. I overlooked the rectifier, that part of it would work as long as you ensure you will never droop below the compliance limitation. Modulating your loop supply is also going to modulate IRET so bear that in mind for the devices that share that common node. Some of your diagrams look like a true 3-wire implementation and I think you would be better off using a 3-wire device like the XTR111 to ensure you don't run into issues, but if you make sure that you keep IRET and loop ground isolated and there are no return paths through any of your other devices, then you should be okay. If you would like a behavioral model to test you can use the attached file. It does not capture the true transient or ac behavior of the device, but is a good dc model to make sure everything is properly connected.

    XTR115_SPICE_ZK.TSC

  • 0 in reply to Zak Kaye
    TI__Genius 11135 points
    Rodrigo

    We haven't heard back from you so we assume this answered your questions. If you need additional help just post another reply below.

    Thanks
    Dennis
  • 0 in reply to Zak Kaye
    Intellectual 280 points
    The best solution i found was dividing in two projects, now i'm using XTR111 in one and XT115 on another, two boards would be a lot cheaper than those isolators. and made it a lot simpler. Fot the display i choose an e-ink, consuming negligible current.