XTR117: XTR117 , 4-20mA current

HI Siavash,

The DAC circuit needs to be powered by the XTR117 +5V regulator, and you will need to ensure that the complete DAC+XTR circuit current consumption is less than 4mA. 

When working with 2-wire current transmitters, there are a few considerations:

• On a two-wire current transmitter such as the XTR117, the IRET potential becomes the local ground for the complete two-wire transmitter circuit, where, the DAC must be powered by the XTR117 regulator, and all circuity must be referred to the IRET GND.
• In general, the overall current consumption of the 2-wire current transmitter (XTR+DAC) must be less than 4mA (or less than the under-scale current).

You could consider using a low-power DAC such as the DAC73xx powered by the XTR117 regulator. It is important that your complete 2-wire current transmitter circuitry consumes 4mA or less current in total, otherwise the transmitter output signal will not be able to reach the minimum scale output at 4mA.   If you need more than 4mA to power the microcontroller or digital output sensor, you'll need to power the sensor externally and isolate the digital connections between the DAC and controller. One possible solution is to use an isolator such as the ISO76xx to isolate between the DAC and microcontroller/sensor, since the DAC is referred to the IRET (floating GND).  In this manner, the digital output sensor can be powered externally by a separate supply / GND.

Please review the following 2-wire current TIPL tutorial videos.  Video 3, focuses on 2-wire current transmitters:

Video1: Introduction to 4-20mA current loop transmitters

Video2: Current loop transmitters configurations

Video3: Analog Input 2-wire 4-20mA Transmitters

Please also review the following 2-wire Current Transmitter frequently asked questions, where these and similar topics are discussed in detail:

2-Wire 4-20mA Sensor Transmitters: Background + Compliance Voltage

2-Wire 4-20 mA Sensor Transmitters: Understanding the 2-wire Transmitter GND

2-Wire 4-20mA Sensor Transmitters: Sensor and Transmitter Current Consumption Limitations (Part 3)

Designing Input Isolated 2-Wire Sensor Transmitters (Part 5)

Please let me know if you have questions. 

Thank you and Regards,

Luis

HI Siavash,

If the digital sensor current consumption is low, where the overall current consumption (Digital Output Sensor +DAC + XTR117) is less than 4mA, then you could power the sensor by the XTR117 local regulator as well and in this case you may not require to use the ISO76XX.

However, if the sensor consumes high current, or needs to be externally powered or referred to a different ground potential, then you will need to use an isolator between the sensor digital output and the DAC digital interface.

Thank you and Regards,

Luis

Hi Luis,

Thanks for your help. In meantime I was doing researching and to avoid the current issue you mentioned above, I come up with this plan. Where I left DAC to have same GND as MCU, and I add differential op-amp to the circuit. Please see the circuit below. 

This should sort out my issue. Am I right?

Thanks again.

Siavash

HI Siavash,

Most two-wire current loop transmitters are used to transmit signals on industrial environments, in some applications for a long distance.  Since the loop supply ground is not connected to the Sensor/ DAC local GND, the potential difference between grounds can be large and it is not well controlled, therefore using a digital isolator between the sensor digital output and DAC input will be a robust approach rather than using an instrumentation amplifier.

This circuit will most likely violate the input common-mode range of the INA333, as the input common-mode range is limited within the supplies range of the of the INA333, where the valid input range of the INA333 is a function of the INA333 supplies, the differential input voltage range, the gain and INA333 reference pin voltage. 

If you don't want to use isolation, an alternative is to use a 3-wire current transmitter such as the XTR111 or XTR300, where all circuitry is referred to the same loop ground potential, and avoid the IRET floating ground issue.

Thank you and Regards,

Luis

Hi Luis,

Thanks a lot for your help again.

I'll definitely consider using the  other chips you mentioned above.

however we manufactured the second schematic I upload above. I'm asking this to learn, In our application the distance is short (lets say 30-60cm). Do you think we will face the problem?

for my next version of hardware do you suggest to add a digital isolator or move to  one of these XTR111 or XTR300 chips?

Many thanks.

I appreciate your help.

Siavash

Hi Siavash,

every current that is flowing back into the "Iret" pin of XTR116 must be sourced by the "Vreg" pin. Otherwise you ruin the internal feedback loop and an error occurs.

See also this thread:

e2e.ti.com/.../xtr116-incorrect-reference-voltage-and-malfunctioning-4-20ma-output-driver

Kai

Thank you Kai,

Hi Siavash,

Let us know if you have questions,

Thank you and Best Regards,

Luis

Hi Luis,

Thanks a lot. I needed sometime for digesting the solution.

Again thanks a lot.

Regards

Siavash

Siavash,

 A possible way is to use a low power DAC such as the DAC8551, and low-power digital isolation between the controller and DAC. 

For example, you could use the DAC8551+XTR116 circuit presented on this 2-wire 4-20mA Transmitter, EMC/EMI Tested Reference Design, and use a ultra low-power, digital isolator such as the ISO7041 between the MCU and the DAC8551 digital interface (CS, SCLK, MOSI).  

Thank you and Regards,

Luis

Hi Luis,

I understood the XTR117 operation. I think I have to move to  XTR111.

in 3-wire 4-20ma circuit, with XTR111, the DAC must power on with  REGF?

Regards

Siavash

Thanks a lot.

Hi Luis,

I appreciate it if you could review my new circuit. Just keep in your mind that main power is coming from 24V and 3V3 (24V to 3.3 V regulator) to turn on the MCU and DAC. This time I'm using XTR111AIDGQT. I didn't power up the DAC with XTR111 regulator because I saw in (XTR111: Review of circuit) that you were fine with it. Also I thought because XTR and DAC are sharing main GND it shouldn't matter. If you think it is not fine I have to use ISO7041 between the MCU and the DAC.

Again appreciate your help.

Siavash

Hello Jon,

Many thanks for reviewing my schematic.

About your comments:

1. My plan is to limit DAC output with SW to MAX of 2V.

2. Do you think 249 Ohm is low resistance with 20mA current for reading voltage as output?

3. Thanks for isolation comment.

4. Sure the value for C48 and C43 needs changing. Which I'll do.

5. I'll add TVS diode. Good point.

I appreciate your comments. 

Many thanks

Siavash

HI Siavash,

The diagram below shows the current loop for the 3-wire transmitter, where the load resistor is located on the receiver.  The XTR111 requires a minimum compliance range limited by (+VVSP – 2V) +VSD required for linear operation of the device and external transistor. In this case, since the XTR111 is powered with +24V loop supply, and the voltage at the load is around Rload*Imax = ~+5V,  the XTR111 and external transistor have more than enough headroom/compliance without issues.  The compliance voltage is explained on Video 3 of the TIPL Current Loop transmitter video series (it is not an issue in this case).

Video4: Analog Input 3-wire 4-20mA Transmitters

Thank you and Regards,

Luis