4-20 mA Signal

An “analog” electronic signal is a voltage or current whose magnitude represents some physical
measurement or control quantity. An instrument is often classified as being “analog” simply by virtue
of using an analog signal standard to communicate information, even if the internal construction and
design of the instrument may be mostly digital in nature. This is to distinguish such instruments
from those making use of no analog electronic signals at all (e.g. wireless or Fieldbus instruments).

The most popular form of signal transmission used in modern industrial instrumentation systems
(as of this writing) is the 4 to 20 milliamp DC standard. This is an analog signal standard, meaning
that the electric current is used to proportionately represent measurements or command signals.
Typically, a 4 milliamp current value represents 0% of scale, a 20 milliamp current value represents
100% of scale, and any current value in between 4 and 20 milliamps represents a commensurate
percentage in between 0% and 100%.