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what is the difference of cover and the ground?

Question asked by echo on Nov 20, 2013
Latest reply on Dec 3, 2013 by echo
Hi 
I use ADS2011. I want to know the eye diagram of  the differential transmission line.  I compare the difference of two cases. The first  case  is that  the upper layer  is my transmission line and the below layer is the cover,and the port is from the ground to the top from 3-D vision. The second case is that the upper layer is the same as the first case and the below  layer is added by myself. 
In the first case, I can see the port's difinition and  the "-" is  "GND ".  I see the  direction of the port and the "-" is sarted at cover layer. So  I think the software thinks the cover layer is ground.But if cover is the ground , then how does the signal transmiss??

In the second case, I add the layer, and the port started at this layer, just like the first case but the layer is add by myself. You can see the second Figure and maybe that will be more clear. I think in the case the layer add by myself acts as the ground layer.

Then I think if I add other two port in the layer add by myself in the second case, then at the schemaic I put ground to this two port , and this is no diffirent with the first two cases.

The last figure is the results of the three cases.

Obviously the eye of the last case is bad and the fall /rise time is longer than the first two cases. And the eye is not so symmetric. 

I  hope someone can help me 

Best wishes!

Echo

Edited by: echo on Nov 20, 2013 8:06 PM  

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