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?

  • \$\begingroup\$ Could you post some example schematics perhaps? \$\endgroup\$ Oct 21, 2017 at 3:58
  • \$\begingroup\$ Like this: google.com.tw/…: \$\endgroup\$
    – Nobody
    Oct 21, 2017 at 5:47
  • \$\begingroup\$ But USB 1/2 didn't require such coupling caps. \$\endgroup\$
    – Nobody
    Oct 21, 2017 at 5:48
  • \$\begingroup\$ see electronics.stackexchange.com/a/274269/146704 \$\endgroup\$
    – dandavis
    Oct 21, 2017 at 7:21
  • 2
    \$\begingroup\$ 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. \$\endgroup\$ Oct 21, 2017 at 8:53

1 Answer 1


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.

  • \$\begingroup\$ 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! \$\endgroup\$
    – Nobody
    Oct 21, 2017 at 9:24
  • \$\begingroup\$ 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. \$\endgroup\$
    – Janka
    Oct 21, 2017 at 9:26

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