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XTR115 5 Volt output...

mel rinn
Prodigy 160 points

Other Parts Discussed in Thread: XTR115

Hi,

We are using an XTR115 driven by a PWM signal from a PIC processor to generate 4-20 mil output in one of our sensors. I have recently noticed a 200 to 300Hz parasitic on the output that causes the current output of the 115 to be unstable underr certain loads. The XTR115 is driving an MJD31C which is attached per the examples in the data sheet. We are not using the 5V regulated output on the XTR115 or the 2.5V output and they are both floating in our application. Through a little trial and error, I noticed that if I loaded the 5Volt output, the parasitic went away and the 4-20 output became stable. Do I need to load the 5 Volt output of the XTR115 to keep things stable under all load conditions? The 200HZ oscillation appears at it's greatest amplitude on the base and emitter of the MJD31C or pin 5 & 6 of the XTR115.

Thanks

  • 0
    TI__Guru 64495 points

    Hello Mel,

    I haven't encountered this issue before, but most customers use the VREG output to power the circuitry that drives the XTR so maybe that's why it hasn't come up.  What provides the input to the XTR?  Have you created another power-source from the loop-supply that returns its current back into the IRET pin?  The amplifier in the XTR must compensate for any current returned into the IRET pin and if it is a transient current, the device must be able to keep up with it.

    Also, you mention that the output is unstable only under certain load conditions, would you please describe these conditions?  

    Like most voltage regulators, some output capacitance/loading is usually desired for proper operation.  Please try placing a 4.7nF capacitor on the output of both and see if the issue is resolved.  

  • 0 in reply to Collin Wells
    Prodigy 160 points

    We have another 3.0 Volt regulator which powers a PIC processor and an RS485 interface IC. The PIC is a 24FJ64 and we use the PWM output to drive the input to the XTR through a filter. I have attached several different 4-20 devices to the output of the XTR with varying results. I will try the 4.7 nf caps on the two outputs and see if that helps.

    Thanks....

  • 0 in reply to mel rinn
    TI__Guru 64495 points

    Hi Mel,

    Thanks for the additional information.  

    I'm a bit concerned with how you're isolating the XTR115 inputs from the rest of the system.  As you probably know, the input circuitry of the 2-wire transmitter must be powered from the loop, return all of it's current back to the loop, and completely float in relation to the loop-supply and loop-RTN.  Do you use a digital or optical isolator to isolate the PWM output of the MCU from the XTR?  Many of my follow-up questions will be alleviated if you would kindly share a schematic with us.

  • 0 in reply to Collin Wells
    Prodigy 160 points

    3660.440-005158-000Rev4.pdf

    Here is a copy of the schematic. Unfortunately, there isn't enough power available to run the MCU and the modbus circuitry from the XTR.

     

    Thanks again..

  • 0 in reply to mel rinn
    TI__Guru 64495 points

    Hi Mel,

    After reviewing the schematic I'm not sure if your use of a 2-wire transmitter is appropriate.  I think you need to create a 3-wire transmitter that includes a local power-supply and GND connection if the circuitry required to read/interface with the sensor consumes more than ~3.75mA of current.  How much current is required from your MCU+modbus?

    In order for proper operation of these 2-wire transmitters, the IRET pin must float in relation to the loop-supply/return and all current used from the loop-supply must return through this IRET pin.  Even though you've used a separate external regulator to generate the voltage for your circuitry the return currents for this regulator must still flow through the IRET pin of the XTR and then out the IOUT pin eventually back to the loop-return.  

    If the current of this circuitry is greater than roughly 3.75mA, the XTR will not be able to regulate the loop down to 4mA for the zero-scale reading.  Are you able to achieve output currents as low as 4 mA with your MCU+modbus active?  

  • 0 in reply to Collin Wells
    Prodigy 160 points

    We are able to achieve 4 - 20 ma with this circuit. The main problem has been stability. I will check the power drawn by the MCU and modbus and get back to you.

     

    Thanks again.

  • 0 in reply to mel rinn
    Prodigy 160 points

    I tried running the MCU and modbus off of the 2.5 Volt reference since the MCU and modbus 3471 are both 2.5 volt devices. The current drawn by these two devices is around 2.7 ma. There is an instability during each modbus access when the 4 - 20 jumps by about 2 ma.    

  • 0 in reply to mel rinn
    TI__Guru 64495 points

    Mel,

    To clarify, is this the same instability that was previously observed or is this a new issue because now you're running things from the 2.5V VREF?  

  • 0 in reply to Collin Wells
    Prodigy 160 points

    It's possible Collin that it has been there all the time, but just hidden by the initial 200 hz problem I initially had. Loading the 2.5 and the 5 volts on the XTR has caused this to go away. We poll the sensor about once a second which is when the 3471 is enabled. The small current jump occurs in step with the modbus access.

     

    Thanks... I appreciate your time...