2-wire 4-20mA transmitters: Controlling 2-Wire Transmitter Outputs with Externally Powered Sources (Part 4)

RE: 2-wire 4-20mA transmitters: Controlling 2-Wire Transmitter Outputs with Externally Powered Sources (Part 4)

Faisal Bala — Sun, 29 Sep 2024 14:25:43 GMT

Hi, I am designing a 0-20v to 4-20mA signal converter for an electric pump. I can't find any reference design for this case.

RE: 2-wire 4-20mA transmitters: Controlling 2-Wire Transmitter Outputs with Externally Powered Sources (Part 4)

Weikang Wu — Thu, 16 Sep 2021 14:55:36 GMT

Is there a PDF version of this article?

RE: 2-wire 4-20mA transmitters: Controlling 2-Wire Transmitter Outputs with Externally Powered Sources (Part 4)

Collin Wells — Tue, 08 Sep 2020 12:46:04 GMT

Hello, The best way to keep systems with externally powered inputs functioning well is to isolate the input and create an "Input Isolated 2-wire Transmitter" type system. As mentioned in the line you quote, if you try to take an input signal that has no GND potential relation to the 2-wire transmitter GND and connect it to the input of the XTR, you're likely to experience common-mode input range challenges and other misc issues. Remember, the externally powered source will need to provide the input current to the transmitter's Iin pin and that current will want to return back to the GND potential of the externally connected source. This usually happens through an earth GND return in your lab equipment, computer's power-supply, etc.

This would work is if the input source is truly inherently floating and self-powered, like from a small battery, in which case you should be able to connect the sensor and 2-wire Xmitter GNDs together without issue, avoiding any of the aforementioned common-mode input range challenges.

RE: 2-wire 4-20mA transmitters: Controlling 2-Wire Transmitter Outputs with Externally Powered Sources (Part 4)

Farhad Kafi — Sat, 05 Sep 2020 11:56:28 GMT

Hello Dear Collin,

Could you please explain more the phrase below :

"Even if the GND pins aren’t connected, trying to connect the externally powered MCU output signals to the transmitter will cause issues because the sensor GND currents will try to find a return path through the 2-wire transmitter."

How this issue could be occur?

Thank you.

RE: 2-wire 4-20mA transmitters: Controlling 2-Wire Transmitter Outputs with Externally Powered Sources (Part 4)

Collin Wells — Thu, 09 Mar 2017 05:21:17 GMT

Hello,

This depends on if your application is to design a 2-wire or 3-wire transmitter. The XTR111 is used in 3-wire transmitter designs which are not discussed in this blog series but are discussed in some related blogs by my colleague Kevin Duke. You are correct that you can remove the isolation if you design with a 3-wire transmitter.

RE: 2-wire 4-20mA transmitters: Controlling 2-Wire Transmitter Outputs with Externally Powered Sources (Part 4)

Shibili Rahman — Thu, 09 Mar 2017 05:16:00 GMT

Hi Collin,

One more query. I just found XTR111, in which all grounds can be same (from my initial understanding). Then I think DAC o/p can be directly connected to XTR11 and linear opto coupler can be removed. So I am confused which is the best approach. My concern is usage of linear opto coupler may leads to loss of accuracy. I am eager to hear your valuable suggestion :)

RE: 2-wire 4-20mA transmitters: Controlling 2-Wire Transmitter Outputs with Externally Powered Sources (Part 4)

Collin Wells — Wed, 08 Mar 2017 18:30:36 GMT

Hi Shibili,

Figure 4 illustrates that isolation is required if the sensor/MCU GND has a voltage potential relationship to the VLoop GND (or is directly connected to Vloop GND). If it does, then shorting the sensor/MCU GND to the IRET pin will short IRET to VLoop GND which as described in the 2nd blog in the series will prevent proper operation of the 4-20mA transmitter.

Your system design sounds appropriate because you're including isolation between the DAC+MCU and the XTR input because the DAC+MCU GND is the same as the VLoop GND. As long as the isolator doesn't require more than ~3.5mA of current then the system should work well.

RE: 2-wire 4-20mA transmitters: Controlling 2-Wire Transmitter Outputs with Externally Powered Sources (Part 4)

Shibili Rahman — Wed, 08 Mar 2017 06:26:33 GMT

Hello Collin,

In Figure 4, why does the Sensor GND and Vloop GND connects through Chassis ? Is there any problem for connecting both grounds in PCB itself ?

In our case, we have an analog voltage (DAC o/p). We need to convert this into 4-20mA loop current. The power for MCU+DAC+all other circuitry were derived from 24V Vloop supply. So GND of MCU+DAC will be the same GND of Vloop. Then our plan is to use a linear opto coupler like IL300 (Vishay) for isolation b/n MCU+DAC and XTR input side. The secondary side of the opto shall be powered by Vreg & Iret. Please give me your valuable suggestion.

Thanks in advance

RE: 2-wire 4-20mA transmitters: Controlling 2-Wire Transmitter Outputs with Externally Powered Sources (Part 4)

Collin Wells — Fri, 06 Jan 2017 19:07:19 GMT

Hello Yilak,

In figure 4 the sensor supply powers the sensor, MCU+ADC, and 1/2 of the digital isolator. The XTR116 VREG connection powers the other 1/2 of the digital isolator.

The only cases where isolation is not required are those where the sensor, MCU, ADC, and DAC are all powered solely between the VREG/IRET pins of the XTR117. If the MCU is trying to communicate with multiple DAC devices+XTR devices, or its power source has a potential relationship to the VLoop supply then you must use isolation. Take a look at the next blog in the series for more information about designing input isolated 2-wire transmitters.

RE: 2-wire 4-20mA transmitters: Controlling 2-Wire Transmitter Outputs with Externally Powered Sources (Part 4)

Yilak Tesfaye — Fri, 06 Jan 2017 18:47:44 GMT

On Figure 4, why does the Sensor Supply need to go to the Isolator and then connect to VREG? I'm trying to drive a uC into a discrete SPI DAC into the XTR117, and I'm somewhat confused about where to place the Isolator and when it is absolutely necessary.