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

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Replies: 23

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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!

  • Hi Matt,

    all the circuitry you connect to the input of XTR116 must be supplied by the XTR116 itself!!! See figure 1 of datasheet. If you want to connect something to the input of XTR116 which is powered by a different supply voltage, you must introduce a galvanic isolation as shown in figure 3 of datasheet. Only by this the XTR116 is able to generate the correct 4...20mA output current.

    Kai

  • In reply to kai klaas69:

    I'm not sure I am following... Are you saying I need to power the digital-to-analog converter off the VREG pin? Most examples I see has that pin disconnected and not used, so this is very confusing here.
  • In reply to Matt Roybal:

    Hi Matt,

    yes, everything you power on the input side of XTR116 must be supplied by the associated VREG pin. See again figure 1 and 3 and read the section "minimum scale-current" on page 6. Try to understand how A1 in figure 1 is working. Consider all the currents that are flowing.

    Kai

  • In reply to kai klaas69:

    Ok, I will re-design this so that the DAC is powered from the XTR116, and I will use optocouplers to get the digital I2C signal into the DAC. Is this the only change that is needed? I can still tie all of the grounds together like I already have correct?

    What about problems #1 and #3? (The XTR116 is outputting incorrect reference voltage, and the reference voltage goes to zero when nothing is connected to the current loop).
  • In reply to Matt Roybal:

    Hi Matt,

    no, you cannot connect all the signal grounds together! Pin 3 of XTR116 is no ground pin, but a current return pin. Again, consider how the currents are flowing in your scheme and the scheme of figure 1 of datasheet: In the scheme of figure 1 of datasheet the current into pin 2 is only flowing through R1. In your scheme this current is also flowing through R1, but then back through R2 and out of pin 3. That's totally different from the intended operation of XTR116.

    Kai

  • In reply to kai klaas69:

    Hi Matt,

    the first two pictures show the correct flow of currents through R1 and R2 of XTR116:

    And this picture shows the improper current flow, when pin 3 is misused as signal ground pin:

    Kai

  • In reply to kai klaas69:

    Hello

    Ok, I see. But what I do not understand is the following: The voltage at pin 4 is dependent of the current in the loop. At 20mA and RL of 200Ohm as example the voltage would be 4V at Pin 4, right? How can a current flow from IIN through R1 to pin 4 then when VIN is smaller than 4V?

    What when I do not connect Pin 3 and do not use VREG and VREF? Then IIN will flow only through R1 to the GND of the loop which is the same as the GND of the control circuit (=DAC).

    Robin
  • In reply to kai klaas69:

    Hi Kai, something I just realized... there is only 1x DAC (8 channel DAC), but there are 8x XTR116... how is the current path supposed to work for that? Originally we were talking about using the XTR116 5V regulator to power the DAC, then the DAC's output goes back to the XTR116... but I am guessing I cannot connect all of the XTR116 in parallel and then into the DAC?

  • In reply to Robin Maeder:

    Hi Robin,

    keep in mind that A1 always tries to keep the voltage difference between pin 2 and pin3 to zero. So, pin 2 is a true current input. And the voltage drop across R1, between pin 2 and pin 4, is "mirrored" across R2.

    When you don't connect pin 3 and do not use VREG and VREF, the driving DAC output tries to find an alternative path for the return current back to signal ground of DAC. But this again disturbs the balance between the currents running to and from the XTR116 (pin 7 and pin 4).

    The only way to make the XTR116 work properly, if you want to connect something to the input of XTR116 which is powered by a different supply voltage, is to introduce a galvanic isolation as shown in figure 3 of datasheet.

    Kai

  • In reply to kai klaas69:

    Hi Kai

    Ok I understand. So as Matt wrote above, even when the x channel DAC is isolated from SPI it is not possible to use multiple XTR115 in "parallel" controlled from 1 DAC, right?

    Is there another component to achieve such a circuit with 1 DAC controlling x current loops?

    Thanks for your help