| 4.1
4 to 20 mA Analog Current Loop
The diagram shown in Figure 11 is an analog 4 to 20
mA current loop. This circuit is mentioned here because
it is sometimes confused with 20 mA digital current
loop. The purpose of 4 to 20 mA analog current loop
is to transmit the signal from an analog sensor over
some distance in the form of current signal. Only two
wires are required to send the analog signal and also
supply power to the sensor. A loop supply voltage (24
volts in Figure 11) is used to power the remote sensor.
The remote sensor regulates the loop current such that
the loop current represents the value of the parameter
being measured by the sensor. A series resistor RL at
loop power supply converts this current to a voltage
that can be used by the electronics to record or distribute
the parameter being measured.
Figure 11. 4 to 20 mA analog current loop
4.2 HART®
4 to 20 mA Current Loops
Figure 12 is another example of a type of 4 to 20 mA
combined analog & digital current loop. This current
loop uses HART® Communications protocol. The HART®
(Highway Addressable Remote Transducer) protocol is
used for SMART remote transducers that are compatible
with 4 to 20 mA analog current loops but also have digital
communications on the same two wires. This is accomplished
by superimposing a two-tone Frequency Shift Keyed (FSK)
digital current signal on the 4 to 20 mA analog signal.
Figure 12. 4 to 20 mA analog current loop with digital
communications using HART protocol
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