Understanding HART Current Lops

An important consideration in HART current loops is that the total loop resistance (precision
resistor values plus wire resistance) must fall within a certain range: 250 ohms to 1100 ohms. Most
4-20 mA loops (containing a single 250 ohm resistor for converting 4-20 mA to 1-5 V) measure in at
just over 250 ohms total resistance, and work quite well with HART. Even loops containing two 250
ohm precision resistors meet this requirement. Where technicians often encounter problems is when
they set up a loop-powered HART transmitter on the test bench with a lab-style power supply and
no 250 ohm resistor anywhere in the circuit:

The HART transmitter may be modeled as two parallel current sources: one DC and one AC. The
DC current source provides the 4-20 mA regulation necessary to represent the process measurement
as an analog current value. The AC current source turns on and off as necessary to “inject” the 1
mA P-P audio-frequency HART signal along the two wires. Inside the transmitter is also a HART
modem for interpreting AC voltage tones as HART data packets. Thus, data transmission takes
place through the AC current source, and data reception takes place through a voltage-sensitive
modem, all inside the transmitter, all “talking” along the same two wires that carry the DC 4-20
mA signal.

For ease of connection in the field, HART devices are designed to be connected in parallel with
each other. This eliminates the need to break the loop and interrupt the DC current signal every time
we wish to connect a HART communicator device to communicate with the transmitter. A typical
HART communicator may be modeled as an AC voltage source (along with another HART voltagesensitive
modem for receiving HART data). Connected in parallel with the HART transmitter, the
complete circuit looks something like this: