XTR116: xtr116 draws high current

HI Ismail,

The expected current consumption of the XTR116 is less than 250uA. It is important to highlight that the XTR116 is a two-wire transmitter. 

Questions:

- In the schematic above, it is not clear if IRET is floating with respect to loop Ground.  Is IRET floating with respect to loop GND?

- How are you powering the DAC and MCU?

Two-wire transmitters require the IRET voltage to float with respect to the Loop-supply GND.  In addition, all the driving circuitry on the two-wire transmitter will be typically powered by the +5V VREG of the XTR116; and all the driving circuitry will be referred to the IRET pin as a local floating “IRET_GND”.  In addition, the total XTR116 + all driver circuitry  current consumption must be less than 4-mA.

Therefore, in the schematic above, the DAC needs to be powered by the XTR116 regulator.  The micro-controller driving the DAC will also need to be powered by the XTR116 regulator and referred to IRET, otherwise the digital signals controlling the DAC will be at a different potential. The overall current of the XTR116 and overall driver circuit has to be less than 4mA.  If the MCU controlling the DAC consumes more than 4mA, then you may need to power the micro-controller with an external supply and use isolation on the digital interface connections between the DAC and controller.

If the circuit is expected to consume more than 4-mA, another possible alternative is to use a 3-wire transmitter. The circuit could be modified to accommodate the XTR111 3-wire transmitter.  On 3-wire transmitters, the GND of the XTR111 is in general at the same potential as the loop GND, and the user is allowed to use external supplies to power other circuits as well as the  XTR111.  In the 3-wire transmitter, all circuits are referred to the same GND (at the same potential as the loop GND). 

For reference, below is an article and a couple of blogs that discuss these concepts on 2-wire transmitters:

4-20mA Signal Chain Basics Article: 
    http://www.planetanalog.com/author.asp?section_id=483&doc_id=564225

2-Wire Transmitter Blogs:

I. 2-Wire 4-20mA Sensor Transmitters: Background + Compliance Voltage
    https://e2e.ti.com/blogs_/b/precisionhub/archive/2014/11/04/two-wire-4-20-ma-transmitters-background-and-common-issues
II. 2-Wire 4-20mA – IRET Can’t connect to Vloop GND
    https://e2e.ti.com/blogs_/b/precisionhub/archive/2014/12/09/part-2-two-wire-4-20-ma-transmitters-background-and-common-issues
III. 2-Wire 4-20mA – IRET < 4mA
    https://e2e.ti.com/blogs_/b/precisionhub/archive/2015/01/13/two-wire-4-20ma-transmitters-background-and-common-issues-part-3

Thank you and Regards,

Luis

The IRET pin has no connection to the GND of the current loop. I designed the circuit by following the forums that they should not be combined.

DAC and MCU are powered by XTR116. However, because the XTR116 draws too much current, I disabled these connections and tried again. However, XTR116 continued to draw more current again. I can't get + 5V from the VREG pin because it draws too much current.

 It also draws more current when I remove the 250R resistor in the current loop and apply 7.5V.

I use BCP56 as the transistor. Is it appropriate?

Regards.

Hi Ismail,

regarding C18, see figure 5 of datasheet!

Kai

Hi Ismail,

Is the issue occurring on several XTR116 devices or just one device?  If this is occurring on one device, is it possible to replace the device?  

If the issue is showing on a few devices, could you please post the complete schematic and PCB layout?

As Kai has mentioned, the reference bypass requires one of the required compensations shown on Figure 5 of the datasheet.  The BCP56 (NPN) should work.

Thank you and Regards,

Luis