Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 7)

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AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 7
This circuit cannot be used.
Usually the current loop is not even returned to the system but is added to the field device’s own power
zero volt reference. Therefore the only available measurement point for the current is the channel
connection. The T8431 and T8432 are not designed to measure voltage with respect to the supply rail,
and so the circuit above is not possible. The solution is to fit a current isolator device (e.g. MTL5040)
which makes it possible to reflect the current in the opposite direction, allowing it to be fed into the
channel connection.
Two, three, four wire connections
The passive field loop shown on the previous page is a two wire device; the 4-20mA loop current is
regulated by the device, and flows out on one wire and back on the other. An FTA is adequate for this
circuit, and the 50mA fuse protects the input against shorts.
24V
Channel
0V
System circuit Active field loop Mirroring isolator
24V
Channel
0V
System circuit
24V
Channel
0V
System circuit Passive field loop