4-20 mA Differential Signal vs Single End


Thread Starter

Mark Teixeira

I need a little education. I'm running various analog field devices to a SLC 5/04. Using a AB 1746-NI8 module I can use channels 4-7 for 'differential' signal sources vs. channels 0-3 take 'single end' inputs. Does this mean that I can run signals into 4-7 from devices using different power supplies? Do I need separate isolation devices?
First it sounds like you need to have another look at your NI-8 manual. If you don't have one you can download it at "www.ab.com ":http://www.ab.com - follow the manuals on line link and look for 1746-in006a-en-p. All 8 channels can be differential or single ended. The drawing in the book just just shows you how to hook up 4 single ended and 4 differential inputs. The 4 single ended inputs in the manual example could just as easily be on 1,3,5,and 7 and the differntials on 0,2,4 and 6, or any other combination, as long as you wire it as shown.

Now the answer to your question is basically yes. Single eneded inputs share a common reference. Differential inputs can have floating references. To illustrate, suppose you have one instrument (say a 3 phase power monitor) with 7 analog signals (3xE, 3xI, 1xP) and one common for the seven signals. You can use single ended inputs on your PLC. Now suppose an eighth 10V signal exists, but it's common is -1/2V below the common of the first input. Now you need differential inputs. If you can tie all your instrument commons together and all your power supply 0V terminals to the same ground point then you can still use single ended even with instruments on different power supplies. Single ended inputs are more susceptible to noise and ground loop problems, so I avoid them where ever possible.
Differential inputs into an analog card of a plc simply means that the input can have its own power supply that is not referenced to a common ground. If you wish you can connect all of the devices using a single power supply, which will result in the differential inputs functioning exactly the same as a single ended input. In reality many end users specify that you must provide differential, isolated analog inputs but end up connecting the commons of a single power supply to all the points, which just results in them wasting their money. Hope this helps.