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USB 3.0 Tx Rx Schematic

In this picture (originally posted in this thread), there are capacitors called "AC Capacitor". Why do I need those capacitors? It is a USB 3.0 SuperSpeed connection, so it would not be an AC-Powered device.

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    \$\begingroup\$ That is not a USB topology that i have ever seen. USB uses a half duplex (single pair of lines) not full duplex (what you have pictured). \$\endgroup\$
    – vini_i
    Dec 11, 2016 at 2:16
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    \$\begingroup\$ Here was the diagram reference, and almost every usb 3.0 connector and device has it's own Tx and Rx, which I know. \$\endgroup\$
    – CodingBear
    Dec 11, 2016 at 2:39
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    \$\begingroup\$ @vini_i you must not be looking at usb 3.0 much then. \$\endgroup\$
    – Passerby
    Dec 11, 2016 at 3:09
  • \$\begingroup\$ @Passerby You are correct, 3.0 is out of my wheel house. \$\endgroup\$
    – vini_i
    Dec 11, 2016 at 12:47

1 Answer 1

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While it's powered by DC, the signals are high frequency differential pairs on transformers to prevent the need for dc coupled devices. The AC capacitors couple the differential pairs between the transceiver and receiver, and managed the dc bias blocking.

See the answer by Some-Hardware-Guy on AC-coupling capacitors for high-speed differential interfaces for a full explanation.

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  • \$\begingroup\$ @BenMiller but that prevents the auto complete recognition of the stack exchange url from working. \$\endgroup\$
    – Passerby
    Dec 11, 2016 at 4:41
  • \$\begingroup\$ @BenMiller there's another way, I'll do it on my computer later. \$\endgroup\$
    – Passerby
    Dec 11, 2016 at 4:52
  • \$\begingroup\$ So because the USB communicates by binary signal (which are made of zeros and ones) and it is removable, it is needed for circuit safety, right? \$\endgroup\$
    – CodingBear
    Dec 11, 2016 at 8:28
  • \$\begingroup\$ The caps are part of the impedance / dc blocking required for the performance at these very fast speeds ... see the link in the answer for more details. Not for safety. without them the link fails. \$\endgroup\$
    – Spoon
    Dec 11, 2016 at 11:42
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    \$\begingroup\$ @Spoon Oh... I must misunderstood that. Thank you for replying me. \$\endgroup\$
    – CodingBear
    Dec 11, 2016 at 12:37

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