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Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Hi,
Is there a correct way to terminate unused data + and - lines on a USB powered device? I am only using the +5v and GND to power a homemade microphone amplifier but am unsure as to what the correct procedure is for devices like this.
Should they be pulled down/up with resistors or can they just be left flapping? The USB cable is about 3 metres long so they probably make for good antennas ...
Registered Member #3792
Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136
Judging from the USB specifications, data lines (D+ and D-) are pulled down (grounded) at the host controller via 15 K resistors, indicating a disconnected state. Since they are grounded and the host does not expect a connection from the other end, i assume that you can use the power lines freely (but always inside the maximum operating power standards - 150 mA / 0.75 W for USB 1.0 and 500 mA / 2.5 W for USB 2.0).
You could probably bypass the data lines with 10 - 20 pF capacitors to the ground cable to be sure that no RF noise can disturb the host controller.
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
@PSCG Thanks. I added the capacitors like you said and so far so good. The power consumption of my circuit is well within limits, in fact the power LED consumes more power than the amplifier!
I also connected a 4.7nF cap in parallel with a 1M resistor between cable shield and circuit ground as noted in a number of USB app note documents for good measure.
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Correctly terminating unused USB data lines
