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Diffrential cable and grounding

mush

Mar 15, 2014
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Hello everybody,

as I unerstand the reason of the HDMI cable uses ground in the defferential pairs is because it uses common voltage (LVDS signal with Vcm=1.2V). On the other hand UTP cable does not use ground because the common voltage is 0 (differential volatges -2.5V and 2.5V). Am I right?

Thank you in advance.
 

Arouse1973

Adam
Dec 18, 2013
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HDMI data is differential pairs and has a + and - signal and doesn't need a ground to work properly. The ground is probably used for the I^2C data and clock lines for DDC. And also for the shields that prevent noise on the data lines. You can also get fibre Optic HDMI interfaces to remove earth loops, so then there would be no ground.
Adam
 

mush

Mar 15, 2014
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Mar 15, 2014
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Dear Adam,

Thanks for your prompt replay. You are right about the HDMI cable the extra pin it's not for ground but for shielding. Know I am confused. I can't understand how the current is propagated through a differential pair. Let's say for example the LVDS signal which has Vcm=1.2V and a swing of 350mV. So, the Voh is 1.375V and the the Vol is 1.025V with respect to where?? Who plays the part of the receiving signal current?

Thanks,

Panos
 

Arouse1973

Adam
Dec 18, 2013
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Don't get so hung up on ground and common. If you have two voltages with different levels then you have current between the two points. Ground is used as a referance for signals in a circuit so they return their return current to a common point. So when say you try and turn a transistor on you need to make the base more positive than something. This something is the emitter for an npn type and does not have to be connected to common, if it was say 5 volts the the base would have to be 5.7 volts for it to switch on.

Adam
 

Arouse1973

Adam
Dec 18, 2013
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Ok maybe not needing a ground isn't quite right how I have explaned it. Current must travel in a loop. The sender must have the same current come back to it. So that's why opto isolaters work. The sender transmits current out and it returns. The other side takes current from its supply and it returns.
Adam
 

Arouse1973

Adam
Dec 18, 2013
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Sorry missed replying to this bit. I was in a bit of a rush. Single ended circuits usually have a 0V reference that all the return current head towards on the way back to the source.

Differential signal are referenced to each other and not a common 0V or ground if you want to use that terminology. The return current is back down the signal wires and not ground or 0V, but to note the voltages must be equal in magnitude but opposite polarity to prevent any current ending up in the common 0V if you have one where both 0V of two pieces of equipment are joined together.
 
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