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I'm looking at two different USB 2.0 hubs which both use the GL850. This is Hub 1:

enter image description here
(source: mydigit.net)

This is Hub 2:

enter image description here enter image description here
(source: mydigit.net)

Both take an external power source but Hub 1 uses four 220uF caps. Does anyone know why a cap is used for each USB connector in Hub 1 but not in Hub 2?

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  • \$\begingroup\$ My guess would be that the capacitors are filters on the +5V line of each USB connector. It could be used to ensure that the output power from the USB ports are a clean 5V (low ripple and noise). It's possible that the one with capacitors has a noisier power supply and needs more filtering. I would be interested to see the output from both supplies on a scope. \$\endgroup\$
    – DerStrom8
    Mar 31, 2016 at 19:42
  • \$\begingroup\$ @derstrom8 Right. I'm actually trying to use a GL850 and make myself a hub, but the datasheet doesn't give an example circuit. I'm not sure if I should use some caps like Hub 1. It'd be so much easier if there was an example schematic. \$\endgroup\$
    – John M.
    Mar 31, 2016 at 19:46
  • \$\begingroup\$ Which GL850 package do you have? \$\endgroup\$
    – DerStrom8
    Mar 31, 2016 at 19:56
  • \$\begingroup\$ @derstrom8 I have the SSOP-28. \$\endgroup\$
    – John M.
    Mar 31, 2016 at 19:59
  • \$\begingroup\$ Consider yourself lucky. Some USB hubs have fewer caps beta.ivc.no/wiki/images/thumb/5/51/Eee_usbhub_second.jpg/… or even no caps hootworld.net/pictures/eee/hub.jpg \$\endgroup\$
    – Passerby
    Mar 31, 2016 at 20:25

1 Answer 1

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Here is the datasheet that includes the schematic for the GL850G SSOP28:

http://www.icpdf.com/GENESYS_datasheet/GL850G-SSOP28_pdf_8319365/GL850G-SSOP28_datasheet.html

The schematic is located on Page 2. The capacitors do indeed appear to be part of a filter between +5V and GND on the USB connectors.

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  • \$\begingroup\$ Thank you! Do you know if there's any aspect of hub designs that needs special attention? For instance, if certain lines shouldn't be too close, ground planes, etc.? \$\endgroup\$
    – John M.
    Mar 31, 2016 at 20:09
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    \$\begingroup\$ There are some comments in the schematic saying that certain capacitors need to be close to certain pins. Beyond that I imagine it comes down to standard high-speed routing. USB data lines should be in differential pairs, keep the traces as short as possible and make sure they're tightly coupled to a ground plane. The impedance of the differential pair should match that of the USB cable, generally around 90 ohms. You can find a calculator for determining the impedance here: eeweb.com/toolbox/edge-coupled-stripline-impedance . \$\endgroup\$
    – DerStrom8
    Mar 31, 2016 at 20:13
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    \$\begingroup\$ Also plan your stackup ahead of time and ensure your manufacturer can handle it. If you do the math for the trace impedance, you'll want to tell the manufacturer to control the dielectric of the board to make sure that your math stays accurate. If they change the stackup even remotely, it can throw off the impedance you calculated and it may not work properly. This may not be a major concern with USB in this case, but it's good practice and you can never be too careful \$\endgroup\$
    – DerStrom8
    Mar 31, 2016 at 20:17

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