[SI-LIST] Re: USB cap placement

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: Ravindra Johari <ravindra.johari@xxxxxxx>
  • Date: Fri, 12 Sep 2008 05:40:11 -0700

Ravindra, A capacitor across Dp and Dn will roll-off the signal.  If you 
know your driver characteristics and how you want to modify the 
response, the value is a straightforward calculation.  Do you mean a cap 
so small that it will only roll off above 240MHz?

I suspect that what you might really be trying to cure is a common mode 
problem or perhaps mode conversion caused by imbalance in your layout.  
In that case if you want to use capacitance to cure the problem, the 
free place to look for it is in the interconnect.  You probably don't 
want to go above 6pF anyway.  The trick is staying balanced.  If you 
can't get what you want from manipulating the interconnect, then 
Johanson Dielectrics goes down to 5.6pF / side in an X2Y which will give 
you two very well matched caps good out to GHz.

Steve.
Ravindra Johari wrote:
> Hi Guys,
>
> We are looking to place a differential capacitance of 0402 package between 
> the two USB differential traces Dp and Dn to improve the signal quality.
> Do you have any suggestion as to what is the best possible place to insert 
> the capacitor pad( near the chip or connector) in the layout and how this can 
> be done
> without any affect to the impedence.
>
>
> Thanks in advance..
> Ravindra
>
> ------------------------------------------------------------------
> 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:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
>                 http://www.si-list.net
>
> List archives are viewable at:     
>               //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
>   
>
>
>   


-- 
Steve Weir
Teraspeed Consulting Group LLC 
121 North River Drive 
Narragansett, RI 02882 

California office
(866) 675-4630 Business
(707) 780-1951 Fax

Main office
(401) 284-1827 Business 
(401) 284-1840 Fax 

Oregon office
(503) 430-1065 Business
(503) 430-1285 Fax

http://www.teraspeed.com
This e-mail contains proprietary and confidential intellectual property of 
Teraspeed Consulting Group LLC
------------------------------------------------------------------------------------------------------
Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC

------------------------------------------------------------------
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:
//www.freelists.org/webpage/si-list

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


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                //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: