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.
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