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XTR115: Failing the field

Muhammad Naeem74
Prodigy 120 points
Part Number: XTR115


We are using XTR115 in one of our product to output 4-20 mA current.
Recently, we have had some products returned from the field where the current loop output has failed and current is permanently wrong.
Our investigation revealed the following:
Loop Voltage (VLoop) = 24VDC, RL = 250 Ohm, Iin = varied from 0uA - 250 uA, Measured current (I Loop)= 33.125 mA
The current stays the same regardless of the load resistor and Iin value but current do vary when the loop voltage is changed. i.e. 19.187 mA @ 10 VDC, 32 mA @ 15 VDC,  33 mA @ 30 VDC
We also measured resistance between Pin 3 and Pin 4 of the XTR115 and found that the measured resistance (R2) is only 15 ohm instead of the 25 ohm as specified in the module block diagram (below) 
We believe due to the changes in R2 value, the XTR115 is not able to regulate the current on the loop?
What we cannot figure out is what has caused the change in R2 value?
What will happen (long term effect) if we short pin 3 and pin 4 of the XTR115?
Please see the circuit snippet below, our brief tests on in-house units have shown that when pin 3 and 4 are shorted together the current in the loop is then limited to 33 mA but when the short is removed the module recovers and starts to regulate the correct current on the loop. But failures from the field are not recovered and output incorrect current permanently.
 

Can you please look into this and advise what could cause permanent damage to R2 or is there anything else in the IC that is causing this behaviour, THANKS.

XTR115 failing in the field

  • 0
    TI__Guru 64275 points
    Hi Muhammed.

    Shorting pins 3 and 4 of the XTR115 shorts out the internal 25 Ohm gain setting resistor which will likely result in the maximum output current as you're observing. We don't have information about the long-term affects of shorting these two pins because that is not a recommended operating condition.

    Regarding the resistor value, the R1 and R2 resistors are trimmed ratiometrically, not to absolute values. Therefore, it's not surprising that the values aren't exactly 25Ohms and 2.475kOhms. That said, we typically say that the absolute values can change +/-20% so a change from 25 to 15 is larger than expected. What does the R1 resistor measure as?

    We have a few questions to help identify the issue.
    1.) Is the C12 capacitor located physically close to the V+ and IO pins of the XTR115?

    2.) If you increase the value of C12 from 10nF to 100nF is the issue resolved?

    3.) Can you probe pin 6 (B) of the XTR115 while the circuit is having issues? Is there an oscillation or other strange waveforms?