4-20mA to voltage

[hrc]

OK...embarking on something new here. Figure industrial control guys would know better than the circuit engineering but I figure its a toss up.

I have several 4-20mA current loop sensors, that I need to read with a micro. First off, for a sensor to be using 4-20mA as its signaling, do you simply connect a voltage source to the sensor (most are two wire devices I guess) and then stick a 100 or 250 ohm precision resistor in series to get the voltage? This is not a hobby project so if its suggested to use a unity gain amp section prior to the A/D thats ok. I need something reliable and robust. Cost is an issue of course so not looking for a OEM device to do this, but something I can put on a board layout.

 

[NWBeaver]

Looks like you are on the right track there, hrc.
In the PLC world we use a lot of loop powered 4-20mA devices. The device is simply wired in series with a voltage source, 24VDC is typical, and an analog input module. A 250 ohm resistor across the analog input terminals then gives you a 1 to 5 VDC input to the A/D.

If you are building your own circuit, the unity gain amp section is probably a good idea, as you will need a high impedance input to the A/D in parallel with the 250 ohm resistor. Another thing to consider would be the use of an analog optical isolator between your input and output circuits.

Hope this helps.

 

[hrc]

Any suggestions on an analog optical isolator? I've used digital before, but haven't really seen or heard of an optical
for analog

 

[NWBeaver]

Sorry, I can't recommend anything, as I haven't been designing circuits lately. I just know from looking at circuit schematics and electronics related websites, that such things are available. They typically use an LED/Phototransister to couple the input to an output.
They are also called optocouplers. There are also isolation amplifiers available, which may be more what you need. Try searching for a existing schematic, or look at the semi-conductor websites for places like Analog Devices, Burr-Brown/TI, Fairchild, etc...