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Original question:

XTR106: Design a 4-20mA transmitter for a 700 Ohm input impedance Load Cell

XTR106: 4-20mA transmitter Load Cell measures external circuit current comsumption?

Pablo Fonovich
Intellectual 765 points
Part Number: XTR106

Hi,

I've designed a 4-20mA transmitter with a xtr106 for a load cell. The problem is that i'm getting arround 13mA with no bridge connected, and when i connect it i get arround 2mA more. With the design, 2mA is the current across the bridge. As i understood from datasheet, the idea is that al comsumption refered to Iret would not appear in the outpu, just the produced by the difference from Vin+ and Vin- of the xtr106, am i correct in this assumption? So i don't know where those 13mA come from.

Any help would be appreciated, y attach the schematics.

Thanks in advace.

Pd: i'm using a Fluke 709 precision loop calibrator for the measurements.

  • 0
    TI__Mastermind 23050 points
    Hi Pablo,

    Your assumption about the IRET current is not quite right, all of the current through IRET actually becomes part of the output current. This is why the total current consumed in your input stage dictates the minimum possible output current and needs to be limited to less than 4mA if you want to achieve 4mA out at 0V in. One thing to double check is that your input ground and loop supply ground do not share a common connection through your input circuitry or any other point in your design. If these points are tied together then IRET will not be able to float to the potential necessary to maintain regulation.
  • 0 in reply to Zak Kaye
    Intellectual 765 points

    Hi Zak, thanks you very much for your answer.

    I double checked my board and i found there was a little short between Io and Iret below the xtr106, my bad.

    However, now i'm getting 4mA all the time (i've already checked for other board mistakes but found none yet). This could be due to the 3.3k resistor Rg, but i think i calculated it right. I have 5V reference on a 700Ohm bridge with 2 1kOhm resistors in series. That gives me 2.7kOhm. With 5V across them, i get 1.85mA, which multiplied by the 700Ohm of the bridge gives 1.3V aprox. So according to datasheet, 1.3V/400uA = 3.250k. Is this right?

    Could also the 1k resistors in series with the bridge output be affecting this?

    Thanks

  • 0 in reply to Pablo Fonovich
    Intellectual 765 points
    Nevermind, Rg was wrong. I forgot to multiply by the bridge's sensivity.
    Thanks