This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

XTR115 Problem: Output offset of 5,2mA

Other Parts Discussed in Thread: XTR115, TPS54062

Hi all,

I got an application problem with the XTR115. I attached the schematics.

The HMAM250 is a pressure sensor with an output voltage of 0.5-4.5V.

The problem is that I do always get an output offset curent of 5.2mA. Even after disconnecting R11 (for offset adjustiment of the 4mA) and disconnection of R12 I will get an offset current of 5,2mA with Vout = 0.5V at pin 3 of the HMAM250.

After the output voltage of the HMAM250 increased above 0,8V (equals input voltage for 5,2mA output) the schematics works correctly . 

Earth (GND) is the 0V of the voltage supply.  

Somebody got an idea?

 

  • Hi Uwe,

    My first assumption is that the quiescent current of the sensor is around 5.2mA which is why you can never create an output lower than that value.  The XTR can not create an output less than the current flowing into IRET because the final output current delivered to the load is actually the current through the NPN + the IRET current. 

    Let me try to find a datasheet for the HMAM250 and I will check to see if this is the case.  In the mean time if you can put a current meter in series with the IRET pin and let me know the current flowing into the device it would be helpful.

    I will report back later today.

    Regards,
    Collin Wells
    Precision Linear Applications

     

  • Hi Collin,

    thanks for your help - and yes you were right. The "real" problem ist that the sensor draws 5.2mA current.

    Looks like I do need another isolated DC supply for my sensor.

    Best regards

    Uwe

     

  • Hi Uwe,

    Thanks for the update, I'm glad we've found the source of the issue.

    It is possible to use a switching DC-DC converter (SMPS) to create a more efficient supply for the sensor device.  The power for the SMPS comes from the loop-supply and the return (GND) of the SMPS sinks into the IRET signal of the XTR device.  Since a SMPS will be more efficient than a linear regulator you should be able to deliver 5.2mA for the sensor while drawing less than 4mA from the loop supply. 

    We recently worked through a similar customer design with success.  Some information regarding the design can be found at the link below:
    http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/t/138328.aspx

    The rest of the issue was handled off-line but I can provide any required details.  To summarize we were able to deliver up to 10.5mA to the load from a 20V loop-supply while only sinking 3.3mA back into the XTR115.  This allowed for a successful 4-20mA output while delivering more than 4mA to the local loop supply circuits. 

    Please let me know if there are any questions.

    Regards,
    Collin Wells
    Precision Linear Applications

  • Dear Collin,

    Thanks for the additional information.

    I think I do need a isolated DC-DC-Converter as I cannot use IRet-pin as GND. My GND needs to be the 0V of my 24V loop supply. The reason is that I might have several of those circuits in  one application. And in industrial cabinets the negative potential of the DC supply is always earthed.

    I thought about getting the power for the DC-Converter from any power supply (not necessarily the loop supply) and connect the isolated 5V side to IRet and 5V of the sensor without connecting the sensor supply to the 5V of the XTR115 at all.

    Thanks again

    Uwe 

  • Hi Uwe,

    I'm concerned that there may be some misunderstandings of the operational principles of a two-wire current loop transmitter.  It will not be possible to power the sensor from a supply other than the loop supply for proper operation of the system.  My suggestion was not to connect your system EARTH GND to the IRET pin of the XTR115 as this would violate two-wire operation fundamentals.  My suggestion was to use a simple buck topology SMPS to convert the loop power supply to the sensor voltage and pass the SMSP return current of less than 3.5mA back into the IRET pin of the device.  This ensures that all power for the sensor + XTR device is derived from the loop supply and accounted for in the control loop and also ensures that the sensor voltage is correctly referenced to the same potential as the XTR115 input circuitry.  The buck SMPS circuit would be local to the XTR115. 

    The SMPS input would be from the loop supply and the return path would go through IRET.  The output voltage would be 5V referenced to the IRET potential.  The input current should be less than 3.5mA (which is returned through IRET) while delivering an output current of up to 10mA to power the sensor and any other required loads. 

    An example of this is drawn below:

    Let me know if you have additional questions.

    Regards
    Collin Wells

  • Hi
    I tried this but i am facing some problem. can you provide me more details on using TPS54062.