A customer wants me to add an expansion board to my industrial computing platform to handle multiple 4 to 20ma loops. Currently, he wants to power the loop and receive data from transducers. I've done a little research, and I've found that typically each receiver has a 250 ohm resistor to turn the 4-20ma range into 1-5V. 12-36V (24V nominal) seems typical for power requirements.
However, the examples for receivers seem way oversimplified. A simple 250 ohm resistor would need to be 6W+ to handle a shorted 36V line, for example.
Here are the problem areas as I see them:
1) Shorting performance. I'd expect the system should handle indefinite loop shorts.
2) Isolation. How important is isolation? I see a note that separate loops powered from the same supply could induce ground loops. This would complicate supporting multiple channels. Currently, I'm ignoring this.
3) Multiple receivers. If they want to chain receivers, 250 ohms seems limiting. Is this still common? Could I use say 50 ohms to reduce the voltage drop? I guess that'd be dropping my noise immunity by a factor of five.
4) Surge events. I think I'll put in a series diode, a parallel diode, and a Bourns surge supression IC per-line.
I'm going to set a goal and say 4 channels of up to 36V loop voltage.
I'm tempted to boost my 12-36V system input to 40V and use a power amp to drop that to the loop current. I could use a power amp like the OPA452 for each channel, which would give short circuit protection and per-channel voltage selection.
Am I overthinking it? Not thinking it through enough? Do I just stick four 250 ohm resistors and ADCs in parallel tied to a 24V source and call it a day? (mostly rhetorical)
Bonus question: What's the best way to transmit 4 to 20ma signals? Should I use something like an AD421?