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I wish I knew how analog communication works!

If you have a device measuring a variable, then analog communication sends a signal proportional to the measuring range. Also, your controller can send an analog signal to command a final control element to alter a process.

Analog once used pneumatic devices with the standard of three to 15 pounds per square inch (psi). When electronic devices became more popular, 3-15 psi turned into 4-20 mA (milliamps) or 0-20 mA. I heard that in the beginning you could find 10-20 mA as well. The first devices didn’t allow you to configure them using a handheld, only local zero and span adjustments.

Today, you’ll mostly see 4-20 mA, although you can set up a device to use the less common 0-20. So how did 4-20 mA become the most popular? Several reasons, such as the following:

  • Zero mA could come from a broken wire or a dead device.
  • The device consumes three mA.
  • Four mA is stable and higher than signal noises.
  • Twenty mA is a multiple of four and low enough to be safe for humans.
  • You can easily convert 4-20 mA to 1-5 volts using a 250-ohm resistor.
  • A range of 0-20 mA could get better resolution, but not enough to counter the other reasons for 4-20.

Check out our other content on analog communication:

Analog and digital communication

Q&A:”Hi, I am new to instrumentation and need to know the difference between HART and FOUNDATION Fieldbus. I mean, the process range usually converts from HART to 4-20mA, but what about FF communication?”.Click here for the expert answer.

4-20 mA loop test

Q&A:”How do I do the loop test? I’m working in a plant startup, and I need to do the loop test on all our 4-20 mA field devices.”Click here for the expert  answer.

Converting 4-20 mA to 1-5 volts

Q&A:”Can a field instrument send 4-20 mA to DCS analog input cards? I read that 4-20 mA converts into 1-5 volts. Will an analog card receive data from the transmitter in 4-20 mA or 1-5 volts?”Click here for the expert answer.


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