Pulse protocols overlaid on current loops

Hi, If you're acquiring a 4-20mA signal across a 250 ohm resistor then conditioning it (filter, scale, etc) with a reasonably standard op-amp
setup, what sort of provisions are likely to be required to ensure that common serial protocols overlaid on the loop won't cause problems? The application under consideration is fairly standard industrial, specified accuracy around 0.5%, response times of a second or so. We don't know which types transmitters may be encountered, so the question can't be any more specific.
Asking the question another way, approx what sort of electrical specs apply to these serial protocols - magnitude, timing? TIA
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On Sat, 21 Oct 2006 17:23:35 -0700, "Bruce Varley"

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?
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"Paul M" <PaulMatWiredogdotcom> wrote in message proclaimed to the world:

No, transmitters have not yet been specified. All we can assume is a common protocol and make the design conservative.
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On Mon, 23 Oct 2006 19:23:31 -0700, "Bruce Varley"

Is anything existing?
Do you know how many resistors are going to be in the loop?
0.5 % accuracy is pretty good for the loop. You need to make sure the sensing device across the resistor is high impendence to get that accuracy. You can put resistors in series to the sensing device that are less than 1/1000 of the input impendence and not effect the accuracy or the serial comm that is on top of the loop. This in conjunction with a cap across the sensing devices input will average the reading without affecting the loop.
You can only put around five sensing resistors in the loop before the total loop resistance becomes too high for the power supply of the transmitter to force 20 ma current through the loop. I am assuming you know this, but it never hurts.
If the current loops are not totally isolated from ground, you will have problems. I would suggest that you insist the loop devices are isolated or have isolators installed between the loop and the devices. AGM makes isolators. I can't stress this enough. I have spent days trying to get rid of stray current in existing loops. I had one that somehow got current from a dawn to dusk light. The loop was connect to a computer and the values were being logged. When I looked at the graph with the operators, it became obvious that the signal was wrong only at night and the error cycled on and off. I happened to spy a dawn to dusk light that would try to start and then the thermal overload would trip the light. Ten minutes later, it would try to restart. Watching the light and the graph on the screen confirmed the connection. The light had ground faulted and was pumping current into the ground. Enough was leaking through to the loop to make about 20% error in the signal.
At least I was working by the hour then.
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