Episode #53: What Is The Difference Between Single-Ended and Differential Signals?

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The Motion Control Show

A very important topic to understand when it comes to signals is the difference between single-ended and differential.  Today we are going to talk about “What is the difference between single-ended and differential signals?”  I am Corey Foster at Valin Corporation.  Reach out to us at this email address here or at TheMotionControlShow.com.  Let's see where we can learn.

Single-ended signals could be analog or digital, and it's just going to have one signal, one wire where that signal goes.  The problem is that if it picks up on any electrical noise, it can see a spike like this or a spike here.  This one is going to take away this pulse.  This one here is going to add a pulse.  So that's a bit of a problem.  With a differential signal though we get two wires, two signals going.  One is going to be the opposite of the other.  Here we have a positive square wave.  Here we have a negative square wave.  If we see any electric noise, it's going to look the same on both of those.  These two wires are typically wound around each other in a pair, so you'll see the wires being twisted around each other.  They are called a twisted pair, so if it picks up noise on one, it is going to pick up noise on the other, and it's going to look the same.  You have a Channel A and then A-not being the opposite.  So then when the A-not is inverted, that electrical noise is also inverted and then they can be compared.  When it's run through a comparator, you get one clean signal.  Therefore, the differential is much more immune to electrical noise than a single-ended signal.  Of course, it probably is going to cost a little bit more in the design.  There's another wire in the cable.  There's more processing now because you have a processor you didn't have before.

Let's take a look at an example here where there is a controller with a line driver going through the twisted pair I talked about.  Here's the drive with the receiver.  This is differential.  You see the line driver and you see the line receiver.  The problem comes when people don't understand which they are going to have.  Maybe you're using this servo drive and you decide to go with a less expensive controller that doesn't have this line driver.  The controller has a single-ended output that just simply pulls this line to ground.  So now how are you going to do that?  Here there are forward and reverse signals, so we're going to have a forward and reverse, but each one is single-ended.  If we look at that same circuitry in the case of this drive, this can be pulled up to 24 volts here.  That’s a 24-volt power supply that can pull it up.  Therefore, it's just pulled to ground at the controller.  It's single ended.  It's going to be a little more prone to electrical noise, but it can work as long as you don't have that electrical noise.

Not all drives, though, can actually be pulled to single-ended in that same way.  Here's another drive that has a differential comparator and also has the same sort of thing on the analog command input, +/-10 volts.  This is Step and Direction or just an encoder input, but this cannot be pulled up with the current flowing through to a single-ended ground the same way that other one can.  Just in case you think that this is just one manufacturer's way of doing it, if we look at another manufacturer with a similar sort of differential input, they show it's an RS422 transceiver.  RS422 is a differential standard that is used.  You might recognize it as it sounds very similar to RS485.  They are very much related.  Here is the command input from another manufacturer as well.  You can see that there is a differential input has some circuitry around it.  Again, this is not at all easy or even possible to adapt to a single-ended input.  It may be possible but is not at all easy.  It would have to be done with voltage dividers and stuff outside of it.  I’m not even sure what that would look like at this point.  It doesn't matter because it becomes messy, becomes something you don't want to deal with, and it would still be very prone to electrical noise.  So that is the difference between single-ended and differential signals.

I am Corey Foster at Valin Corporation.  Reach out to us if you need help.  We are always happy to help.

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.