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Collin Wells

The previous three parts of this blog series focused on 2-wire 4-20mA sensor transmitter designs composed completely of analog components. While analog signal conditioning is practical for linear sensors, many sensors have nonlinear outputs that can only be modestly corrected with analog compensation. Examples of non-linear sensors include pressure sensors, gas and chemical sensors and many temperature sensors. Therefore, high-accuracy designs require advanced math or lookup tables to convert the sensor output into an accurate analog linear control signal for the 4-20mA transmitter. In these systems, the sensor output is acquired using an analog-to-digital converter (ADC) and then processed in a microcontroller (MCU) before being converted back to an analog signal to control the transmitter.

Figures 1 and 2 show two simplified 2-wire transmitter designs that use an MCU with an integrated ADC for sensor acquisition and digital processing. Figure 1 displays a high-accuracy design using a 16-bit digital-to-analog converter (DAC), such as the DAC8411, to convert the processed sensor signal into an accurate analog control signal. Figure 2 uses the MCU pulse-width modulated (PWM) output and an external low-pass filter to create a cost-effective, but less accurate DAC.


Figure 1: 2-wire sensor transmitter with MCU+ADC and DAC

  


Figure 2: 2-wire sensor transmitter with MCU+ADC and PWM DAC

 

As I explained in my previous posts, the MCU and DAC must be fully powered between the VREG and IRET pins of the XTR116, and the IRET pin must not be connected to the VLOOP GND or to any other external voltage potentials. Also, the circuitry powered from the loop must consume less than 3.8mA so that the XTR116 can properly output the 4mA zero-scale level.

While the system designs shown in Figures 1 and 2 are common, you will occasionally have to design a system using an MCU powered from a voltage source referenced to another external potential, possibly VLOOP GND, to control the output of a 2-wire transmitter. Figure 3 shows an example of this type of system.


Figure 3: Issues with controlling a 2-wire transmitter with an externally powered MCU

 

In this type of system, the sensor and MCU can’t be directly connected to the DAC or PWM filter circuitry. If the sensor and MCU are directly connected, the 2-wire transmitter GND pin (IRET) will be pulled down to VLOOP GND, or another voltage potential, preventing the transmitter from functioning properly. 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.

Therefore, as shown in Figure 4, you must use a digital isolator between the sensor and MCU circuitry and the 2-wire transmitter circuitry. Integrated isolation solutions such as the ISO7310 can accomplish the digital isolation. However, be sure that the digital isolator doesn’t consume more than the allowed current budget for the 2-wire transmitter.


Figure 4: Isolating an externally powered MCU from the 2-wire transmitter with the ISO7310 digital isolator

 

In summary, if you’re trying to control the output of a 2-wire transmitter from an externally powered control source, be sure to include a digital isolator between the controller and the 2-wire transmitter circuitry to prevent the grounding issues I described in detail in Part 2 of this series.

In Part 5 of this series, I’ll discuss the design of 2-wire sensor transmitters that require the sensor circuitry to be powered from, but also be completely isolated from, the rest of the 2-wire transmitter. These systems are commonly called “input isolated” or “sensor isolated” 2-wire transmitters.

Related TI Designs reference designs for 2-wire transmitters:

http://www.ti.com/tool/TIPD126

http://www.ti.com/tool/TIPD158

 

Related 3-wire blog posts from my colleague Kevin Duke:

Find commonly used analog design formulas in a new pocket reference put together by Art Kay and Tim Green.

Anonymous

  • Is there a PDF version of this article?

  • 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.

  • 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.

  • 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.

  • 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 :)