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XTR106: Output shifts with number of UUT.

Part Number: XTR106

We have 4-20mA pressure transducers with a typical XTR106 recommended board. If we put one transducer in the test stand, we get a nominal 4mA output at zero pressure. If we add a second transducer, the first one's output may go down or go up 40 to 80uA. The direction and magnitude are transducer dependent. Adding a third transducer usually increases the up/down shift. Why does adding a transducer change the output? We found this when using MCC USB-2416 DAQ, or just using two handheld DMMs. The plot below starts with 3 transducers in test, then one, then two, then three, and repeats the sequence starting with a different Serial Number.

  • Hi William,

    You mention adding units but it isn't clear exactly how you are connecting the devices together. Can you show a schematic showing your connections?

    Regards,

    Mike

  • Mike,

    Our system is connected essentially like the 2-wire configuration of your video: Current Loop Transmitters, #2, Current loop transmitter configurations. Our DAQ inputs connected across the 250 ohm resistors.

    Thanks,

    Bill

  • Bill, 

    You must be referring to the image shown below:

    Your issue seems related to the grounding between units.  I'd like to verify, are the XTR grounds completely separated?  The sensors to do share a common ground or chassis?  

    Regards,

    Mike

  • Mike,

    The chassis grounds (Iret) in the figure with the red oval and line through it, are not connected to chassis ground by design. There is a ground plane on the board which is mechanically tied to the cap/element (the mechanical fixture that the strain gages are attached, and a pressure port, stainless steel). Each transducer is attached to a manifold (stainless steel), with the cap/element threaded port screwed into the manifold. The manifold is in contact with oven (grounded, stainless steel) port and attached with a conductive pressure hose, so it is contact-wise connected to ground.

    Note, I also get zero shifts when adding transducers in a bench configuration with just a power supply and handheld DMMs. A DMM reads the voltage across a 250 ohm receiver resistor.

    Thanks, Bill

  • Mike,

    I confirmed on the transducer the Iret is not connected to ground. In doing so, I notice the marking on the XTR106 had "BB" in a square, and "17AFSPK" above the "XTR106U". I assume that is still your part?

    Thanks, Bill

  • Hi Bill,

    Yes, I confirmed our devices still have the "BB" logo on them.

    When you showed data for zero current above, why is it ~4.9 mA and not ~4 mA?  I assume this is a value you would expect for a "zero" reading from the transducer, which means that the transducer or the transducer stimulus could be part of the reason for these shifts. The only way to rule out the transducer side is to short the input.  Do you still see the shift if you disconnect the transducer, and short VIN+ to VIN-?

    At this point I can't be sure if this is related to the output current stage or that the input transducer configuration may be changing.  In addition to a schematic showing the configuration, it would be good to get voltage measurements for:

    1)  supply voltage

    2)  VIN+/-

    3) VREF 5, 2.5

    4) IRET

    Regards,

    Mike

  • Mike,

    The units I was testing were not calibrated, and the pressure was incorrectly set at MEOP + 14.7. I have other cases where they are calibrated, the correct pressure, and the current is at 4mA +/- 20uA, and we get the same shifts.

    I'm not sure what you mean by "Do you still see the shift if you disconnect the transducer, and short VIN+ to VIN-?" Do you mean disconnect the bridge gages (sensor) and short pins 5 & 2 on the XTR106?

    I assume the measurements you're looking for above are the pin names on the XTR106? What is the reference point to measure from?

    I sent Matt Gloekler the schematic.

    Thanks, Bill

  • Hi Bill,

    I'm going to move this support offline, I've already sent an email, please let me know if you did not receive it.

    Best Regards,

    Mike