[SI-LIST] Re: Current Return Path

This is very interesting.  I wish all the rules of thumb and common 
knowledge could be supported by/put to the test with such experiments.

My ideas on this one:

I am assuming that vterm is the exact same supply as vdd of the driver.

1. Removal of the bypass caps will make the return path impedance 
higher.  The effect of this may be visible more as increased power suppy 
noise due to switching of several signals.  This noise can also couple 
on to victim nets.

2. Thus, I think looking at vterm is also important.  The variation in 
vterm should be higher after removal of caps.

3. Finally, depending on the edge rates, on-die bypassing may have been 
sufficient.  (I am not sure.)

Abhijit.


kfrobinson@xxxxxxxxxx wrote:
> All,
> I was recently evaluating a board design and noticed that some signals =
> were referenced to a power plane(VTERM) and some nets were referenced to =
> ground.  This is a very large board 13" x 22" and there are bypass caps =
> between the power plane and ground plane around the perimeter of the =
> board.  I thought I would see if I could destroy the current return path =
> by removing all the bypass caps between the power plane and ground plane =
> except from the far end of the board furthest from the driver.  I then =
> looked at the signals that were referenced to the power plane and they =
> were identical to what they had been before the removal of caps.   I =
> took scope shots at multiple locations before and after the cap removal. =
> The net in question runs the length(22") of the board right down the =
> center.  I had expected to see a pronounced change, but did not see one. =
>  Any thoughts?
> 
> Stack up
> s
> vterm
> s
> s
> vcc
> gnd
> vcc
> gnd
> s
> s
> gnd
> s
> 
> Ken Robinson
> kfrobinson@xxxxxxxxxx
> 
> 
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> 
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
> 
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> 
> List archives are viewable at:     
>               http://www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages 
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>   
> 
> 



------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: