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Does anyone know why AC coupling caps should be implemented in USB3 interface while it's not required in USB1/2?

Anything to do with transmition speed, encode method?

Marcus Müller
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Nobody
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  • Could you post some example schematics perhaps? – Kevin Brant Oct 21 '17 at 03:58
  • Like this: https://www.google.com.tw/search?q=usb+ac+coupling&rlz=1CDGOYI_enTW688TW688&hl=zh-TW&prmd=ivn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjSw_3rgYHXAhUHppQKHSMWDVwQ_AUIEigB&biw=375&bih=591#imgrc=NgJjxxoHTqYAZM: – Nobody Oct 21 '17 at 05:47
  • But USB 1/2 didn't require such coupling caps. – Nobody Oct 21 '17 at 05:48
  • see https://electronics.stackexchange.com/a/274269/146704 – dandavis Oct 21 '17 at 07:21
  • I know AC caps are for DC bias blocking, my question is why USB1/2 don't need such caps. – Nobody Oct 21 '17 at 08:35
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    The USB1/2 has a totally different transmitter, with a very small DC bias. More, some protocol states (suspend, resume) use DC levels. The USB3 uses completely balanced differential dignaling with LVDS-type transmitters, and thus they need the AC decoupling. – Ale..chenski Oct 21 '17 at 08:53

1 Answers1

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The question has to be why these caps aren't there in the D+/D- lines.

The D+ and D- lines of USB (any, also USB3) cannot be capacitively coupled, because an USB1 host detects the low/full speed of the connected device by checking whether D+ or D- is connected to 5V through a 1.5kΩ resistor. The caps (if any) are inside the device, behind that 1.5kΩ resistor.

Keyboards, mice and game controllers often are low-speed devices to allow cheaper cables and controllers, so these are still common.

USB2.0 uses the same D+/D- pair for communication, so it keeps the DC coupling into the device.

This requirement isn't there for the additional RX and TX pairs of USB3.0.

Janka
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  • So, in USB3, on the D+/D- lines, caps still not needed. Only the new added TX/RX lanes need AC caps, right? What's the role of D+/D- and TX/RX play in data transmission? Thanks! – Nobody Oct 21 '17 at 09:24
  • You cannot place caps into the D+/D- lines, only directly before the device controller, if that device controller you use requires it. Same for the TX/RX pairs. You have to look into the device controller datasheet. – Janka Oct 21 '17 at 09:26