Do Differential Pairs Need Ground? Are you sure? | Explained by Eric Bogatin
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- Опубликовано: 18 май 2024
- When doing PCB layout and designing boards, many people ask if GND is important for differential pair signals. Here is the answer. Thank you Eric.
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Chapters:
00:00 What is this video about
01:16 P & N
02:09 Real differential pair vs. two single ended lines
07:11 Differential pair going through a transformer vs. ground
08:35 Are diff pairs routed on board different from diff pairs in cables?
11:42 Differential vs. common
15:12 What if a differential pair doesn't have any return plane - examples explained
17:28 Simulation of a single ended signal vs. return current path
21:05 Simulation differential pair signals vs. return current path
24:04 Tightly vs. loosely coupled differential pairs
28:51 Differential pairs vs. return plane far away
34:03 Example 1: Single ended signal in cable
34:56 Example 2: Single ended vs. differential signal in cable
36:22 Results: Impedance graphs
39:51 Example 3: Single ended vs. differential signal in PCB without GND plane
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Bogatin’s so clear in his thinking and presentation. I think the key is how clearly he separates concepts and breaks problems down to the most basic principles. He’s a pure pleasure to listen to 👍😁
I took a Signal and Power integrity course at my University that followed Bogatin's -Simplified book. I learned so much and everything was so accessable and walked through concepts that were possibly missed from earlier coursework. Fantastic speaker, fantastic teacher, fantastic writer. Thank you for helping us access this fountain of knowledge
Your videos with Eric are always extremely useful.
I agree, I learn a lot
Amazing thank you both for educating us for free
Thank you Eric and Robert!
Awesome content! Can you cover the case of less than 1MHz for us mixed signal/audio folks?
“A wire sitting in space”. Lol. Also known as an antenna?
Great presentation
I could listen to the professor speak all day he’s just wonderful
You guys are awesome. I can learn abour the most important topics in a circuit that didnt even mentioned in schools ans university's
Excellent!
24:04 there are two factors pointed out:
1) DC resistance
2) tolerance
but how about actual resistance of the signal against EMI? Is it not affected by pair lines being farther apart?
Videos like this are the reason I'm a Patreon supporter. Many thanks for doing this.
Thank you very much
Another maybe obvious point of view is that if we are driving some receiver, those have some limits on common mode voltage (voltage of shorted diff line to IC ground) they can handle. In that case we still need DC ground to be at least routed somehow, even if we have ideal diff pair, otherwise we have two floating inputs with some differential voltage between them.
Eg. for CAN bus between to battery powered devices wee need ground (or luck).
If we are not driving input transistors in some IC but simple coil as in ethernet transformer than we can "delete" the ground as utp cable.
Are erics slides for download somewhere? I'd like to read through the rest of his presentation.
Excellent presentation on differential pairs!
One question about return currents: In your example with a twisted-pair cable carrying differential signals, the fields cancel out within the cable. But what happens when the differential pair transitions to a PCB with a return plane nearby? Eric Bogatin says that most of the return current will be coupled to the plane.
If the PCB return planes of the two connected devices aren't directly tied together, wouldn't the return current struggle to complete its path? In that case, wouldn't it be better to connect the planes to ensure controlled coupling and minimize EMI issues?
Thanks.
Can you please explain what happens to voltage and current in long wires? If I apply 24V , how long can the wire be? and what happens for signals
What is the difference between odd mode impedance and differential impedance? Also, removing the ground plane would not work for even mode?
A bifilar transmission line is a differential pair?
Someone please help me understand what the return current does at the interface where the cable enters/exits the PCB? If we have RS422/485 differential signals on Shielded Twisted Pair, connecting to a PCB with no direct connection of grounds between the two systems, what happens to return current at that interface? It can follow the ground plane in PCB up to the connector, and the differential twisted pair return current is on the other wire in the H/L pair. But at that interface where the return current in the PCB has no where to go, is that similar to what Eric was saying it will give a convoluted return path but they'll be equal/overlap and cancel out?
I still missed one thing. In the case where transformer drives diff line (e.g. UTP Ethernet cable) - where will the single ended signal come from? Isn't the function of the transformer to eleminate the SE signals?
Imagine there is a length difference between the two traces from the transformer to the Ethernet jack so the two sides of the pair are not perfectly out of phase. The net (common mode) signal at any given point in the pair is now an AC wave at your signal frequency. Your board, traces, connector, etc. have capacitance to ground for the return path. At least, that is my understanding 😅
@@jessicav2031 for my simple mind length difference is a good enough explanation. It makes sense to me. Thanks.
Very useful. Why does Shielded Twisted Pair cause EMI issues? Can someone explain? I think it has to do with common signal return path?
10:25 ? Who said shielded?
thanks man, i somehow heard shielded and was surprised lol@@dzidmail
I think this is because, the differential signal will have a clean return path (the difference ie., D+ - (D-)), but the common single ended (avg.) would actually not have a return path and would oscillate as antenna (without reference) and thus causing EMI issue.. Anyone can feel free to comment and carry this discussion.. Thanks!!
What does Eric say at 3:06? All differential pairs are two single-ended transmission lines? Edit: ok yes I was right as you can see later on the slides but it's really hard to tell as he seems completely swallow the word "ended".
How can I get the full course from Mr Bogatin? It is so interesting!
I'm not certain about that but he does has a couple books. They electronically link to videos for visual illustration of the point at hand.
Thumbs up for rocking the brass rat
So the secondary of a transformer that is not center taped to the ground is still a two single ended pair? If you feel this doesn't sound right and smells bad, you acre correct, because it's a terrible generalization.
That configuration sounds single ended to me
coaxial screen grounded one end (source)!
Why in the hel would you prevent Eric from talking??????