[SI-LIST] Re: USB grounding issue
- From: Jory McKinley <jory_mckinley@xxxxxxxxx>
- To: "SSantangelo@xxxxxxxxxxxx" <SSantangelo@xxxxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 8 Aug 2013 14:06:54 -0700 (PDT)
Hello Steve,
You potentially have unwanted coupling between your high speed USB current
return, chassis ground and digital ground if I understand your design
correctly. Under the Edge Fingers of your USB device I understand that you
have chassis ground shape this is where you get your potential coupling path.
I would use a separate grounding structure under your USB Edge Fingers which
then ties to your chip/controller. This ground will tie to the chassis but
through Beads. Be careful where you place the grounding structure in the
stack-up as the impedance discontinuity is important, you want to hit 90ohms
differential so you will want to simulate this structure with a 3D solver.
You probably do not have easy access to your active device(s) but using a VNA
or Scope on your various grounds you could pick up the spectral noise coupling.
You should be able to setup a relatively quick simulation that shows you where
your coupling is occurring and then you can see where by breaking the chassis
ground to digital ground connection will reduce the coupling noise.
Regards,
-Jory
________________________________
From: "Santangelo, Steven" <SSantangelo@xxxxxxxxxxxx>
To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
Sent: Thursday, August 8, 2013 1:22 PM
Subject: [SI-LIST] USB grounding issue
Hi All,
We have a externally facing High Speed USB port and are experiencing what looks
like a grounding issue. We have collection of memory sticks where some work
and others don't. It appears that the sticks that tie the shield/connector
housing to digital ground on the stick do not work reliably while the sticks
that don't make this connection work fine. We also verified that if break the
chassis to digital ground connection by insulating the housing of the failing
sticks with kapton tape we can get the failing sticks to work just fine.
I've read several recommendations on how to handle ground on this interface but
remain a bit confused since they differ in their approaches. In our system we
have a shielded USB connector with the shield fingers contacting the chassis
face. We also have a chassis ground shape under the connector with the shape
tied to chassis ground via a plated mounting hole (as well as the press fit
connector shield pins). Two high voltage 0.001uF caps are tied between the
chassis GND shape near the connector and digital ground. Both the power and
GND signals on the USB connector (pins 1 & 4) pass thru ferrite beads and there
are 2x120uF caps plus a 0.001uF cap across pwr and gnd on the connector side of
the beads. Lastly, the USB signals are connected to a pass-thru protection
device as well as a common mode choke.
The component placement is of course important but I was wondering if the
overall approach is one that has been used successfully by others. Also, has
anyone experienced this type of grounding problem and how was it resolved?
We're in the process of removing the protection device on the data signals and
moving the power filter caps closer to the connector but any additional
thoughts on other things to try or measure would be greatly appreciated.
Thanks
Steve Santangelo
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