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long distance 4-20 mA has issue

Mustafa Gulec
Intellectual 540 points
Other Parts Discussed in Thread: XTR111, XTR117, XTR115

I have a device that has 4-20mA output, referenced to circuit ground, which I used XTR111 to realize the transmitter. This device works without problem in most situations. But I got reports that in some situations, it has some issues. When the receiver(a plc) is connected to the transmitter over 30-40m shielded cable (grounded on the receiver side) in an industrial environment the output increases suddenly about 1-1.5mA at the receiver side and oscillates a bit.. I did not inspect the site yet but they say plc has a 220V supply which is a EH150 Hitachi plc. Also they say when the connection to the plc is severed the situation goes away. They also say, there is no voltage or anything at plc analog input. I've tried with 100m cable with different type of receivers with seperated ground with the transmitter in the office and did not face any problem. What may be the issue? Does using current loop transmitter like xtr117 may improve the situation? May isolating the mA improves the issue? Thanks.

  • 0
    TI__Guru 63225 points

    Hello Mustafa,

    4-20mA loop drivers are intended to communicate over extremely long distances and the 30-40m of cabling is not uncommon, so while I'm not immediately sure what's going on in their system I have a few suggestions. 

    It sounds like there may be something going on when the shield is connected to the GND on the receiver side of the system.  Does the system behave normally when the shield is left floating or is grounded on both sides? What potential is the shield connected to relative to the GND on the XTR111?  

    Since they're using a shielded cable I'm guessing they operate in a noisy environment, is it possible that they're seeing some type of higher frequency energy in the system being rectified down to a dc offset?  Both radiated and conducted emissions can cause dc shifts in the XTR output.  If this is the case then 2-wire transmitter topologies (XTR115/67) are typically more immune to both conducted and radiated emissions.

     

  • 0 in reply to Collin Wells
    Intellectual 540 points
    Hello Collin, thanks for the suggestions which some of them I thought of too. But the site is far away and I could not inspect on my own yet. But interestingly another device which has similar function and has 4-20 output has no issue in the same setup. I believe it's analog output is isolated and it may use current loop. There is also one thing about current loop that I can't be clear. Since the receiver and the transmitter is isolated (or transmitter is not isolated, like in my case)and the receiver side has nothing to do to supply power to the loop, the power should be supplied at the transmitter side I believe. So would not it be affected again from the environmental noise? Also what differs when we connect the receiver or left it float? Because the problem arises when the receiver side is connected.... By the way the shield is connected to the EARTH on the recevier side not to any gnd of any power supply.
  • 0 in reply to Mustafa Gulec
    TI__Guru 63225 points
    Hi Mustafa,

    The situation you've described above where the system works with a 4-20mA transmitter which has an isolated output and local power-supply sound like a 4-wire 4-20mA transmitter. Is it possible you're trying to interface with a 4-wire current loop receiver? If that's the case you'll need to design a 4-wire transmitter. If you provide me the model number of the analog input module you're trying to interface with I can take a look to help understand what type of transmitter and possibly isolation is required (2-wire, 3-wire, 4-wire).

    I have four blogs out there on designing and understanding the operation of 4-wire transmitters which may be helpful to you:
    e2e.ti.com/.../4-wire-current-loop-sensor-transmitters

    I also have 6 blogs out there on designing 2-wire transmitters: e2e.ti.com/.../two-wire-4-20-ma-transmitters-background-and-common-issues

    Part 2 discusses the issues if the IRET signal on the XTR is not left floating and is connected to a potential relative to it. Earth may have some relative potential to your transmitter which is possibly why it causes issues.
  • 0 in reply to Collin Wells
    Intellectual 540 points

    I inspected the device that has an analog 4-20mA output that works without problem in the same environment and it has an isolated analog output but is not current loop. That device has 220VAC power supply input and analog output section has 10V isolated and regulated supply. The mA- output is same with the gnd of the 10V isolated supply. So it is not a current loop.  The receiver side is also isolated but I don't exactly know it's input circuit (

    http://www.hitachi-ies.co.jp/english/products/plc/eh_150/index.htm

  • 0 in reply to Collin Wells
    Intellectual 540 points
    So it is 2 wire transmitter.
  • 0 in reply to Mustafa Gulec
    TI__Guru 63225 points

    Hi Mustafa,

    Actually based on your description, the transmitter is definitely a 3 or 4-wire transmitter which is why your 2-wire transmitter circuitry doesn't work.  2-wire transmitters receive power from the loop and transmit the signal back in the form of a 4-20mA output.  Therefore, 2-wire receivers provide the loop-supply to the transmitter and measure the current the transmitter returns.  4-wire receivers do not provide the loop-supply on the same wires as the output current so if you plug a 2-wire transmitter into a 4-wire receiver input channels then the 2-wire transmitter will not be powered.  I suggest you go through the 2-wire and 4-wire blog posts that I previously referenced to gain a better understanding of how these systems look and how to connect to them.

    Here's the 4-wire transmitter posts again for reference:

    e2e.ti.com/.../4-wire-current-loop-sensor-transmitters

  • 0 in reply to Collin Wells
    Intellectual 540 points
    I have isolated the 4-20mA and solved the problem. Thanks for your suggestions and helpful tips.