FreeLists / si-list / [SI-LIST] Re: Common mode current is USB bus powered devices cabl e

["Clewell, Craig" <Cclewell@xxxxxxxxxxxxx>]

Bharath, 

You should see some common mode current in the USB cable.  There is a
specification on the amount of common mode you can have in a USB
application.  Check the website at www.usb.org  Look for the Cable and
Connector class document.  This PDF will define what the common mode
tolerance needs to be.

CC

-----Original Message-----
From: Bharathkumar Raju [mailto:bharathrajupcb@xxxxxxxxxxx]
Sent: Tuesday, February 06, 2007 20:31
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Common mode current is USB bus powered devices cable


Dear Experts, 
                   I recently tested some (about 5 different types) USB
devices (USB 1.1 Full speed devices with data rate 12 Mbps, low bus powered
devices) for understanding the common mode current flowing through the
cable. To verify and ensure the balance in the current flowing through the
Vbus conductor, ground conductor and shield conductor I measured the nett
current flowing through these 3 conductors using a current probe.
  At regular intervals there is about 25 milli amps common mode current
flowing in both the directions. The current pulse is of square wave with
about 80 nano-seconds duration. Is this common mode current in DC +ve, DC
-ve & Shield conductors of an USB cable is a normal behavior or am I missing
something in the measurement. 
  The cable contains 2 twisted pairs, one pair is the D+ & D- signal wires
and the other pair is Vbus and ground (DC - ve). In all the devices I found
that ground and cable shield are directly connected in the device PCB.
  I request the experts to throw light on this
   
  Bharath
                                
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