When would I use a 4-20mA differential current input?

Question

I've recently run into a scenario where I need to pick between a singled ended current input (4-20mA) input device and a differential current input device. Single Ended and Differential. I've never run into (or so I believe) the case where I had a differential current output sensor, thus I've never looked into an I/O card that accepts a differential input current signal.

So my question - when would I know if I needed to use a differential input card? It seems like most 4-20mA transmitter circuits operate in a 2 wire interface. Is the two wire interface always considered to be single ended? Thus the only time I would ever need to think about using a differential input card for a current output, is if there was a non-two-wire interface?

I guess I'm more familiar with the concept of single-ended and differential in the context of voltage I/O, not current I/O.

Answer 1

For both voltage and current I/O, single-ended circuit have one of their wires connected to a known potential (GND or power), while in the differential circuits both wires may be at arbitrary potentials. Thus, any transmitter with 2 wire interface may operate in either mode (with the exception of externally powered/grounded device with non-isolated outputs)

Differential inputs has higher common-mode noise immunity, higher immunity to noise on power lines, and lower voltage on wire (the datasheet claims <10V common mode for differential, while single-ended will have one wire at 24 volt)

Single ended inputs are easier to make, so they are likely cheaper; you can also simplify wiring by sharing a single common wire between multiple sensors (but this will make sensors more noise sensitive). On the other hand, if the wire detaches from sensor and contacts ground (say because you did not tighten the screws properly), you short your 24V psu to ground, which is a bad thing.

Answer 2

In differential mode, the signals are independent to each other. They have their own reference. When a single power source is used for all end device instruments (with a mix of loop and differential signals) wired to an I/O card, then differential signals could be wired as single ended.

When Isolation transformers of any manner are used, then the reference voltages may be floating (absolute voltage zero volt / referenced voltage circuit potential). If such is the case then this presents a problem. The requirement in single ended to tie all commons together could be inducing a potential between the commons. This is potentially (ha ha) going to induce an inaccurate reading due to unpredictable reference levels of the returns.

In such cases, one can still wire single ended as differential. It simply requires the common of the power source to be used as the reference or return leg.