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I designed a device on which there is a HUB, FT232 , two external USB port and some periphery.

I wanted to check the noise on the power bus. And I saw only white noise. enter image description here

But after I connect the USB cable, I see on ALL circuit this is an noise. enter image description here enter image description here

Most likely it is common mode noise from fronts D+/D-, because if I connect only the VBUS and the GND noise disappears.

How terrible is this noise?

How can i get rid of this noise?

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  • \$\begingroup\$ Correct probing is important. Please make your measurement as shown in this image: e2e.ti.com/cfs-file/__key/… \$\endgroup\$ – Rohat Kılıç Jun 26 '18 at 15:33
  • \$\begingroup\$ What amplitude is it? \$\endgroup\$ – Andy aka Jun 26 '18 at 15:35
  • \$\begingroup\$ I measured as shown in the bottom picture. \$\endgroup\$ – AndreyB Jun 26 '18 at 15:35
  • \$\begingroup\$ Andy, about 200mV \$\endgroup\$ – AndreyB Jun 26 '18 at 15:36
  • \$\begingroup\$ Rohat, I measured it again as in the picture below. And the noise disappeared! But I do not like that my circuit generates such noises. \$\endgroup\$ – AndreyB Jun 26 '18 at 16:01
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Here is my take at this "USB problem".

First, the noise, as shown, is a typical noise from an on=board DC-DC switcher, with ~400 kHz switch rate, which has no relation to USB signaling. It is very likely that this is an emitted noise from switcher magnetics and/or poorly-designed high-current loops. The noise might be real, or might be just a loop pickup on sloppy scope probe connection, and it might have no effect on actual functionality. Here are some very useful references on ho to deal/avoid switching noise in DC-DC converters, from TI appnote, and from "Power Electronics News"

Why there is no spiky noise when only power is applied? When a USB design is powered up but no D+/D- activity exists, all USB IC fall into "SUSPEND" state by design, and therefore consume little-to-no current. When power consumption is low, modern switchers usually turn into some pseudo-LDO mode, and don't switch. My guess is when D+/D- are connected and all on-board USB ICs wake up due to USB traffic, they begin consuming current, and the switcher starts switching and produces these EMI spikes.

First, you need to check your measurement technique, and use a correct one: enter image description here

Second, check your on-board switcher if the layout is done in good accord with manufacturer's suggested plan.

Then,... it depends.

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Maybe at Your test suite You can't observe any bad behavior after connecting the USB cable, but some day someone will connect Your device to her/his PC with very bad and ugly power supply... In EMC high immunity is as important as low emission.

Good practice in designing USB and similar serial interfaces is to use common mode chokes specially designed for fast data lines. Below You can see such common mode choke together with ESD supression and single choke on supply line.

enter image description here

Example common mode chokes for USB are WE-CNSW series from Wurth or DLM series from Murata.

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