Hi,
I have an adjustable output voltage on a circuit and our customer has requested implementing a 4-20mA transmission line with this.
Technically the adjustable voltage could create an adjustable current using a fixed load resistor, but thought it would need to be more sophisticated than this and a Current Transmitter would be need to measure the current on the line to provide a feedback loop.
Any advise would be greatly appreciated.
Best Regards,
Dave
There are specialized 4-20mA transmitters. 2-wire transmitters like the XTR117 allow the circuit to be powered from the loop; 3-wire transmitters like the XTR111 require a separate power supply. You could also implements these circuits manually.
Hi Dave,
A 2-wire transmitter uses a "floating ground" as a current return path. This requires the input circuitry to be powered from the XTR voltage regulator and the input ground to be disconnected from system ground and instead connected to the Iret pin of the XTR. See diagram below. In the 2-wire configuration the XTR and input circuitry are limited to a 4mA current budget, because the signal and supply current share the same loop.
As your input circuity has already been designed, you may prefer to implement a 3-wire transmitter such a the XTR111 that Clemens recommended. The 3-wire transmitter allows a common ground to be used for the input, XTR, and receiver, and you will not need to change your existing supply to the input circuity. See in the diagram below, the supply current and signal current do not share the same loop.
Here is a link to our FAQ on XTR 4-20mA transmitters: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/945188/faq-designing-with-4-20ma-current-loop-transmitters-xtrs-faq-links?tisearch=e2e-sitesearch&keymatch=xtr#
Some of the FAQ's links to our TI Precision Labs videos are broken, so here is a link to the videos covering XTRs: https://www.ti.com/video/series/special-function-amplifiers.html
Thanks,
Zach
