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In Reply to: RE: USB isolation hub (Gordon, audioengr, John - input wanted) posted by pryamomimo on February 22, 2008 at 07:59:59
Since we are into the subject, what about removing pin 1 ( power) from the cable at the PC end. Since there is some noise in the PS wouldn't it be beneficial to not have it running in the cable under the shield next to our data line?
Bob
www.PlateauLight.com
AFAIK, most of USB chips (at least the ones involved here) need both the differential data pair, AND the power lines for device identification, even if running in self-powered mode.
I think it is Gordon who uses a different TI USB chip at the DAC end that needs just the differential data pair. I guess your suggestion might work in this case.
Actually even the 2706/7 can run fine without the actual +5 volt wire on the cable in most circumstances.
You HAVE to have the ground wire though. You still need a voltage reference (ie ground) for the differential pair. As I said before putting chokes etc in line with the ground is a bad thing to do as well. Some designs do that to try and meet EMI specs, but all it can do is mess up the quality of the data transmission.
John S.
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