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XTR111: Current drops with increase of loop load

Leonardo Trinta
Intellectual 310 points
Part Number: XTR111
Other Parts Discussed in Thread: PGA309

Hello there

I have a XTR111 based, 3-wire 4-20mA transmitter on a custom converter and I found a problem with resistance load's of loop current higher than ~60R. The problem is: With resistance higher than 60R, the current drops to ~2.8mA instead 4mA, and ~5mA instead 20mA. Without 60R load it works properly. 

Can anyone help me? It's urgent. 

Below the schematic. (VIN is the same at anytime). 

Best Regards,

Leonardo Trinta

  • 0
    TI__Mastermind 49405 points

    Hi Leonardo,

    Can you kindly provide the following information to help debug the issue:

    1)  Is the 60Ω load resistor referred to the same GND potential of the XTR111? What is the voltage at the XTR111 circuit output (header P8, pin 1 with respect to the XTR111 local GND when the issue occurs?  Please confirm with a voltage meter the voltage right at pin 6 (VIN) when the issue occurs?  Kindly confirm the voltage supply VSP with external meter is also at 24V.

    2) Please share the complete circuit at the XTR111 input?  What is the VREF voltage?  

    3) How many XTR111 devices/PCB boards show this problem? If the issue only occurs in one device, can you please replace the XTR111 device and confirm if the issue persists. 

    4) If you wish, we can review your Gerber plots/PCB layout as well.  

    A general suggestion is to replace the OD pull-down 100k resistor with a smaller 10kOhm resistor. This is likely not related to the issue, but a general recommendation to ensure the OD is always well below the low-level threshold.

    Thank you and Regards,

    Luis

  • 0
    Guru 140200 points

    Hi Leonardo,

    what else do you power with "VREG"? Keep in mind that the "REGF" pin can only source 5mA.

    Kai

  • 0 in reply to Luis Chioye
    Intellectual 310 points

    Hello Luis

    Thank you for your reply.

    1) Yes, the load is on the same GND potential of the XTR111. Voltage supply and VSP are the same all the time. Just current output changes. The voltage across header P8 increase as the load resistance increase (it begins at 0.04V up to 0.768V at 60R or higher resistances). 

    2) Yes, I can send it to your inbox. VREF Voltage is a voltage reference generated by PGA309. 

    3) It occurs at all boards that we tested (5 boards). Because it depends of impedance of measurement circuits, just now one of our customers related this issue and we can reproduce it in laboratory. 

    4) Sure. I'll send to you inbox.

    Thank you for the suggestion. I'll do it in the next release.

    Best Regards,

    Leonardo Trinta

  • 0 in reply to kai klaas69
    Intellectual 310 points

    Hello Kai

    REGF just supply an PGA309 chip but thank you for your observation. I'll check if my wheatstone bridge has low resistance.

    Best Regards,

    Leonardo Trinta

  • +1 in reply to Leonardo Trinta
    Guru 140200 points

    Hi Leonardo,

    Leonardo Trinta said:
    The voltage across header P8 increase as the load resistance increase (it begins at 0.04V up to 0.768V at 60R or higher resistances).

    Hhm, one pn-junction's forward voltage drop... Are you sure D4 is mounted properly? This unidirectional TVS would behave exactly that way when being reversely connected by mistake...

    Kai

  • +1 in reply to kai klaas69
    TI__Mastermind 49405 points

    HI Leonardo,

    I agree with Kai, can you please remove diodes D4 for debug purposes?

    Diode D4 CDSOD323-T24S is a unidirectional 24V working peak voltage, where pin 1 should connect to IOUT and pin 2 to GND.  However, looking at the schematic symbol and layout, I can't immediately tell which is pin 1.   As kai suggested, It may be worthwhile to check if it is mounted properly.

    Thank you and Regards,

    Luis

  • 0 in reply to kai klaas69
    Intellectual 310 points

    Hello dear

    Thank you for observation. Really we had an issue in our BOM. The part number CDSOD323-T24S of D4 diode needs "C" after T24S, without this, it is a unidirectional diode and it is the mistake. 

    Unfortunately it was assembled with inverted polarity hahaha.

    Thank you very much for help. The issue has fixed and our customer was happy. 

    Best Regards,

    Leonardo 

  • 0 in reply to Luis Chioye
    Intellectual 310 points

    Hello dear

    Yes. As I described in my answer for Kai, it was the issue and was fixed. 

    Thank you for your help.

    Best Regards,

    Leonardo Trinta

  • 0 in reply to Leonardo Trinta
    Guru 140200 points

    Hi Leonardo,

    good to hear that it was only a placement error Relaxed

    Kai

  • 0 in reply to kai klaas69
    TI__Mastermind 49405 points

    Hi Leonardo,

    That's good news. Thank you,

    Luis