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As of now, I have a fairly good understanding about the 4-20mA current loop standard.

It needs, of course, a sensor with either 2-wire or 3-wire output 4-20mA (4mA as zero reading and 20mA full-scale output).

A Power supply that must be sufficient for the input voltage of the sensor input and voltage drop across a 220Ohm Resistor.

Then lastly, since analog current passes through the resistor, the analog voltage drop can be obtained by a microcontroller via ADC.

But I noticed in my research that only one sensor is being used.

I want to use 2 different sensors with 4-20mA output. How do I start with this? Should I use two different loops for each sensor? Can I just use 2 different analog input channels?

Thank you.

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    \$\begingroup\$ Each sensor needs its own loop. \$\endgroup\$ – Janka Mar 24 '17 at 0:54
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    \$\begingroup\$ Each sensor needs it's own current loop AND 2 different analog input channels. \$\endgroup\$ – BobU Mar 24 '17 at 1:59
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The best way to understand this is to look at wiring diagrams from actual vendors.

For instance, Acromag has such a diagram, if you look here p7/8 you will see that your 2 loop transmitter can share the same 24VDC power supply but need 2 different input channels.

I have extracted here for illustration and in case the link goes dead later:

Figure 7: Multiple Transmitters Sharing The Same Power Supply (credits: Acromag)

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  • \$\begingroup\$ Hello, thank you so much for this. So I think it is ok to say that all the sensors can share the same supply, and there can only be one single controller for the analog input channels. However, some sensors are already 2/3-wired so they already functions as transmitters. So in this case, is it ok to remove the "TRX"s in the diagram? \$\endgroup\$ – J. A. De la Peña Mar 27 '17 at 7:19

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