[SI-LIST] Re: Common mode current is USB bus powered devices cable

["Evans, Jeff (Analog Engineer)" <jeff_evans@xxxxxx>]

Bharath,
USB contains and intentional CM signal called EOP (end of packet).  This
is why CM filtering impedance on USB signal pairs is limited to 60 ohms.
Best regards,
Jeff Evans


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Bharathkumar Raju
Sent: Tuesday, February 06, 2007 5:31 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Common mode current is USB bus powered devices cable

Dear Experts,=20
                   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.=20
  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
  =20
  Bharath
                        =09
---------------------------------
 Here's a new way to find what you're looking for - Yahoo! Answers=20

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 =20

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• References:
  • [SI-LIST] Common mode current is USB bus powered devices cable
    • From: Bharathkumar Raju

Other related posts:

[SI-LIST] Common mode current is USB bus powered devices cable

[SI-LIST] Re: Common mode current is USB bus powered devices cable