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I am working on a USB 2.0 High Speed Data device that will have a power input filter for EMI and overall power integrity. The filter includes inductors on both the +V and GND. So this means there will be high frequency impedance between the GND for the host and the device. Does anyone know what kind of issues this might create with High Speed Data signal integrity, what I should be considering to make sure the data rate doesn't get compromised?

Thanks

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There's a lot of parameters there.

In principle a USB connection uses differential signals, which means all signal current should flow only in the data wires.

But usually affordable USB devices are designed in the mind-set of a solid ground connection, which means there could well be small transient spikes that relate back to the data signals.

While it is likely those spikes are so small that a moderately well designed system has the appropriate capacitance to catch them.

More to the point, you are introducing a purely reactive element to one of two connections (D+/- and Power) in a dynamic system, so you will need to be extremely careful about oscillations due to plug/unplug, power transients and coupled noise as well.

Because once your device starts oscillating (which you may not even notice at first) compared to the supply, it may go up and down so far, that your data-lines effectively get pulled into corners that your host can't handle. Because your D- and D+ will always be driven between the V+ and GND of your design, so if your GND wobbles up and down by 10V, you are likely to break your computer. Even if your V+ stays neatly the same distance from GND all through that wobble.

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USB is only sort of differential, it has a single ended zero state, so adding impedance in the ground is a bad idea, I wouldn't do it.

The usual issue with USB is that the USB ground is not necessarily the same as the ground connected to whatever external device you plug in, you see this a lot with sound cards where noise as the USB ground moves around with the processors C state is fairly common. The cure is NOT to mess with the USB power, but to do the isolation after the USB has been converted to something more reasonable for on board use, put the isolators the other side of the chip doing the USB comms.

Regards, Dan.

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