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  • TI Thinks Resolved

XTR116: Incorrect reference voltage and malfunctioning 4-20mA output/driver.

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Replies: 38

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Part Number: XTR116

Hello, we created an 8 channel 4-20mA output/driver based off the XTR116. We are experiencing three problems:

1. The datasheet clearly states that the XTR116 has a VREF of 4.096 volts. The chips we received are correctly labeled as XTR116, but they are outputting 2.5 volts and not 4.096.

2. No matter what current we provide on the IIN pin, we get roughly 40mA on the current loop, it doesn't change. We have a digital-to-analog converter providing voltages/currents to the XTR116 to control this.

3. We are using the VREF off the channel 1's XTR116 for the digital-to-analog converters reference voltage. If we don't have anything hooked up to this current loop, the XTR116 no longer outputs a voltage on VREF... it becomes zero.... which causes all of the other 7 channels to fail.

Here is the circuit schematic showing the 8 channel 4-20mA drivers, the digital-to-analog converter for setting IIN, and the power supply. The idea is that any device that receives a 4-20mA signal can just hook up to 2 of the 4 terminals labeled as ISOLATED_POWER, and ISOLATED_GND. The power supply is already tied into the current loop:

Any help or advice on these 3 problems would be greatly appreciated, as we already have PCB's of these made, thanks!

  • In reply to Robin Maeder:

    Hi Robin,

    yes, "using multiple XTR115 in "parallel" controlled from 1 DAC is not possible", unless you provide a galvanic isolation.

    You could add a single channel DAC to each input of the XTR115 and use optos in the digital control lines.

    Kai
  • In reply to kai klaas69:

    Hi Kai, thanks again for the help.

    I put in the 8x DAC for 8x XTR116, am using a I2C multiplexer, and using an optocoupler for digital isolation, does this look correct?

    This DAC (MCP4725) does not have a reference pin; it uses its VDD as the reference. Because of this I am powering it directly off the VREF pin on the XTR116 since its more accurate than the VREG pin. It was not clear in the datasheet how much mA this pin could support, but it seems to be able to at least support this DAC and pullups? It sounded like as long as the DAC and pullups don't exceed 4mA then it should work, and I can leave VREG disconnected.

    p.s. This adds a lot of extra components to the PCB. Are you sure there is no way to just use a single 8 channel DAC for 8x XTR116's? You mentioned it might be possible with "galvanic isolation", but I am having a difficult time imagining how that would work?

    Thanks again for the help and advice!

    -Matt

  • In reply to Matt Roybal:

    Hello Matt,

    I believe your best option is to use a single DAC for each XTR116 and control their interfaces with optos, since this is a 4-20mA 2-wire transmitter and each DAC driver circuitry is intended to be powered by the XTR and the DAC potential is referred to the IRET pin of the XTR116 they drive; as it was suggested by Kai on the previous post. The reference accuracy is specified with 0mA to 2mA load.

    If the 4-20mA current loop application allowed the use of 3 -wire transmitters, an option is to implement voltage to current converter using the XTR111. Please refer to figure 36 of the XTR111 datasheet; where all the circuitry can be biased with respect to the current loop ground.

    Thanks and Regards,

    Luis
  • In reply to Luis Chioye:

    Hello Luis

    With the XTR111 I can use several ICs in "parallel" to control multiple loops with one DAC, right?

    Best regards
  • In reply to Robin Maeder:

    Hi Robin,

    any non-loop powered, 3-wire voltage-to-current converter allows this.

    Kai
  • In reply to kai klaas69:

    Is that an acceptable solution though? The schematic I posted above with the 8x DACs?

  • In reply to Matt Roybal:

    Hello Matt

    If you want to use the XTR116 your schematic with 8 DACs is the only working solution. With a 3-wire transmitter you can do it with just one DAC but the function is a different one. Check e2e.ti.com/.../920511 for the difference between 2-wire and 3-wire loops.

    Robin
  • In reply to Robin Maeder:

    Yes but what about only using the VREF (4.096v) to power the DAC and not the VREG in my new schematic?
  • In reply to Matt Roybal:

    Hi Matt,

    you might also want to read this:

    Kai

  • In reply to kai klaas69:

    Hi Kai, thanks for the article. Isn't this what I implemented above? And to re-iterate my question again, is it ok for me to power this stuff off the VREF pin instead of the VREG pin? (leaving the VREG pin disconnected). Because as I said above, the DAC doesn't have a reference input voltage.... and the VREF on the XTR116 will be more accurate than the VREG pin. I will post my schematic again:

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