20Ma Current Loops - Intab PC-Logger 2100 User Manual

3.4.2 20mA Current Loops

Current loops can be connected in a number of ways. They are basically the same
but appear to be different because they are powered and/or referenced to gnd
differently. Below are a few examples with illustrations.
NOTE : The shunt resistor must for current measuring be manually connected
via a small jumper (=short wire) between "I" and "+". See schematic diagram of
inputs in section 3.3.1.
1) Connecting current loops with common ground points:
NOTE! Ground loops can give rise to considerable measurement errors !
Try to keep other instruments in the source path of the transmitter. This will
make it impossible to exceed the common mode range. You can of course also
connect them in the return path ( ground side ) if you make absolutely sure that
the common mode range (10V) isn't exceeded.
Add resistances of inserted instruments to make sure. The sum may not exceed
10V/20mA=500Ω !
2) Connecting ungrounded (floating) current loops:
Again it must be stressed that "floating" is meant in respect to "2100" signal
ground, not any other ground that may be lying around (like safety earth/ground)!
This, of course, also does not necessarily mean that it is floating with respect to
other grounds! Confused? - Get an electronics engineer to help you!
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INTAB Interface-Teknik AB, Sweden
Avoid ground
loops by only
connecting the
gnd of one
channel to system
ground.
Give all floating
current sources a
ground reference by
shorting the
negative inputs to
"2100" signal
ground with the help
of a short piece of
hook-up wire.