XTR115: Current output drops off when external ground is connected to board.

Ryan Gibson

Part Number: XTR115

Tool/software:

Hello,

We have implemented the XTR115 in an industrial product providing position feedback over a 4-20mA loop. Iin is fed directly by an MCU pin providing a variable voltage according to a sensor reading whose value is provided to the MCU over serial connection. This circuit has worked fine in almost all situations, however when an external ground connection is made to the board connecting the loop current drops to zero regardless of the input signal. Vreg and Vref are left disconnected due to the signal coming directly from the MCU. The 24V supply for the current loop is isolated to that loop only.

Problem sequence description:

1. Device is operating normally, outputting 4-20mA corresponding to sensor position

2. External ground is connected to the board, for example an RS232 connection. The RS232 ground is connected to AGND1 via an inductive filter.

3. 4-20mA loop current drops to 0.

4. A 0.1V potential was measured between Iin and Iret, where previously the potential was 0V.

Has anyone else experienced an issue like this?

Best regards,

-Ryan

3 months ago

0 Luis Chioye 3 months ago

TI__Guru 54536 points

HI Ryan,

Yes, as you correctly mentioned, the 2-wire current transmitter XTR115 IRET local ground needs to float with respect to the 4-20mA current loop supply. The sensor or circuit that is driving the XTR115 needs to be referred to the same IRET potential, and in most cases, the XTR powers the driver circuit. The total current consumption of the Sensor + Driver Circuit + XTR must be less than 4mA on a 2-wire 4-20mA application.

Ryan Gibson said:
This circuit has worked fine in almost all situations, however when an external ground connection is made to the board connecting the loop current drops to zero regardless of the input signal. Vreg and Vref are left disconnected due to the signal coming directly from the MCU. The 24V supply for the current loop is isolated to that loop only.

1) Please share complete XTR115 schematic, showing the MCU connections to the XTR115, and showing the MCU ground and power connections.

2) Is the external RS232 GND connection completely isolated from the 4-20mA loop supply GND connection?

One of the most common issues with 2-wire current transmitters occurs when the IRET potential is unintentionally shorted with the 4-20mA loop-supply GND.

At the bottom of this post, please find links to a blog/article discussing 2-wire current transmitters. The IRET requirements are explained on part 2 and part 3. Part 4 and part 5 of the blog series show show examples of MCU and sensor circuits connected to the 2-wire XTR through isolation.

Please see the two figures below, where a MCU PWM is fed through an isolator, and a RC low-pass filter to generate the current input signal into the XTR.

Figure 1: Shows issues when controlling a 2-wire transmitter with an externally powered MCU: