XTR111: Current out is not tracking voltage in

Hello Andy, 

Thanks for posting on e2e and welcome

Please confirm the following for me: 

1. What is the voltage applied to "4-20 PWM"? I understood Pin 6 of the device is receiving 0 to 3.3V. 
2. What is the voltage applied to "VIN_5-24V"? If this is lower than 8V, the minimum recommended power supply for the XTR111 is 8V. 
3. When the output current measures around 39mA, what is on OD and EF? 

All the best,
Carolina

Carolina,

1. The voltage applied to 4-20 PWM is a 0 - 3.3 VDC PWM signal coming from an ESP32 micro.  At 0% duty cycle the voltage at pin 6 is 0V.  At 100% duty cycle the voltage at pin 6 is 3.3V.  The RC filter formed by R34 and C29 create a direct relationship between duty cycle and voltage over the 0 - 100% : 0 - 3.3V range.

2. The applied voltage as VSP is 24VDC.

3. OD is being shorted to ground via a dip switch, so 0V.  EF# is reading High (1) by the micro.  I did not physically measure EF#.  I used the value returned by reading the pin on the ESP32 that EF# is connected to.

Thanks,

Andy

Carolina,

Some additional information that might help.  I tried another prototype and everything was working correctly, at first.  I was getting a proper 4 - 20mA output based on the PWM input I was giving it.  However, after about an hour of testing, suddenly the current output when to 33mA.  I only noticed this because the output was feeding a test PLC 4-20mA input and the current reading had pegged and was not moving.  I disconnected from the PLC and monitored the current with a multi-meter and that is when I read 33mA.  The only thing I was doing at the time of the "failure" was adjusting the 24VDC supply to 23VDC to see if it had an effect on the 4-20mA output.  Unfortunately, I never got to take a reading as the part failed while I was adjusting the power supply.  The EF# pin continued to read high by the micro.  After writing this, I tried to take some additional readings on 4-20mA line and for some reason it is now reading 1.6mA and is not moving no matter what I put on the PWM line.

Thanks,

Andy

Hi Andy,

That behavior sounds a lot like there was a current-limit situation occurring. I've attached the portion of the datasheet that I'm referring too. 

1.6mA is now all it would output? Did this occur only after a power cycle? Are you seeing a change in the quiescent current before and after this behavior? 

I'm wondering if something is happening on the power supply, like a huge transient spike, that might be causing this problem. 

Best Regards,

Robert Clifton

As of this morning, I am no longer getting any current output from the device at all.  The problem occurred while adjusting the output of the 24VDC power supply from 24VDC to 23VDC.  The circuit was powered at the time of the adjustment.  I have not been closely monitoring the quiescent current.

I went back to a prototype that was working correctly to continue my testing.  The output was tracking the PWM input very well for almost an hour then I started to notice a negative offset building on the output.  When I run a 3.3VDC signal into the XTR111, the current is no longer 20mA.  It has dropped to 19.84mA.  My resistors are all 0.1%.  Unclear if this offset drift is related to my problem above.

Thank you, Robert.  I will make a change to the FET.