4 to 20ma current loop, driven by data?

xanatos

Has anyone done something like this:

I have to take a digital signal (which I will receive serially in the form of a fat data stream containing 4 bytes of information that I will actually be using that contains the sensor's reading), and take those 4 bytes and turn it into a 4 to 20ma current loop output. I'm guessing D to A converter and some sort of translation circuitry... but not exactly sure what - or if possibly there's already a chip that does this automatically! :-)

Thanks!!!

Dave

TinkersALot

do you mean like one of these ?

http://circuits.linear.com/382

http://www.eetimes.com/design/power-management-design/4010357/PWM-controlled-4-20-mA-current-loop-transmitter-provides-galvanic-isolation

tonyp12

i guess you need to create a analog voltage to feed one of these:

Current Sense Amplifier
http://www.mouser.com/ProductDetail/Texas-Instruments/XTR117AIDGKT/?qs=sGAEpiMZZMuo%252bmZx5g6tFDzAJ8Ms8VOK

or a more advanced one
http://www.mouser.com/ProductDetail/Texas-Instruments/XTR108EA/?qs=sGAEpiMZZMuo%252bmZx5g6tFKUde1JFeBe6

Erik Friesen

TI makes some nice parts for 4-20. Are you needing to power the device from the loop?

Phil Pilgrim (PhiPi)

The circuit I presented here should work, depending upon the accuracy you require. The accuracy will depend upon how well-matched the CTRs of the three optos are. Of course, a mismatch might be something that can be calibrated out, too, as long as it's consistent over temp.

-Phil

xanatos

Wow - some cool stuff and good circuits, although I'm gathering that this is going to be a bit more complex on the 4-20 side than I had initially envisioned. Right now, the TI XTR117 (http://www.ti.com/lit/ds/sbos344c/sbos344c.pdf) which shows a digital input driving the XTR117 via a D/A converter and a handful of other parts, is in the lead. Not too bad. I still need to find out exactly what I'm interfacing to - all I know is it's 4-20ma current loop right now.

Thanks everyone - any other brainstorms, feel free to share! :-)

Dave

xanatos

Hmmm... this is kind of cool: http://www.l-com.com/item.aspx?id=6981

An RS-232 to Current Loop Converter! I could extract the sensor data from the other junk it sends and just pass the sensor data to this device...

There's so much cool stuff out there now!

Dave

Erik Friesen

The XTR117 works for loop powered, but not so easy for external powered. If externally powered, the xtr111 is better.

xanatos

Erik Friesen wrote: »

The XTR117 works for loop powered, but not so easy for external powered. If externally powered, the xtr111 is better.

Not sure on that yet, good to know. I don't get much call for current loop stuff, but it's proving to be quite interesting!

Dave