On Sat, 21 Oct 2006 17:23:35 -0700, "Bruce Varley" proclaimed to the world:
Why don't you filter the resulting voltage drop across the resistor and not the current loop itself? Doing it this way will not effect the capacitance or reactance of the loop.
I did a quick search for the capacitance limits on RS-484 loops but did not find any specs. I know I have them somewhere because this has come up in the past when I had problems with a loop. The problem then turned out to be a ground loop in a component that was not fully isolated, not filtering, but I remember measuring loop capacitance and digging up the spec somewhere.
I am cleaning out office files right now. I might run across the old paperwork in the next day or so. I did a bunch of field tests on loops then, trying to get a feel for how robust a loop like this really is. The comm speed made a big difference. Do you know what that will be and what protocol will be used?