USB device cable shield connection - grounding it or not?

Discussion in 'Digital Circuit Design' started by colinb, Aug 30, 2011.

  1. PSoC Pro

    New Member

    Apr 28, 2016
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    You find there's no hard and fast rule. Why? Because a dongle plugged into a PC has different grounding requirements than a high current device like an external HDD, or a MP3 player with amplified speakers. A dongle should be just fine with the shell connected to ground (it's plugged into a grounded connector after all, effectively making the shields one. A device at the end of a long cable will have more bypassing needs. There's no "magic" one-size-fits-all solution.

    The schematic with the many series-parallel components is making a fool of someone(s). Note the 3.3 ohm resistors. With a 5V (nominal) supply and 500mA load, what's the voltage drop. Practical? Why is that resistor even there? To make a fool of someone(s).
     
    endolith likes this.
  2. ioannes

    New Member

    Jul 7, 2016
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    I simply think the path from shield to device logic ground is a damned sure way to get mess inside.
     
  3. elix-cz

    New Member

    Jul 28, 2015
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    PSoC Pro:
    Of course, if I would like to make a fool from others, then only I alone would be a fool. USB bus is not primarily intended for industrial environment but can benefit from its practices, which are represented by series of EMC strandards IEC 61000, mainly 61000-4-XX. In these standards there are categories of device immunity A to C, while A category must operate with applied interference without any degradation of specified functionality. In my professional practices I didn’t develop another device’s class then the A. The common problem of some not thorough EMC solution is, that they improve immunity only on some frequencies while on others they strongly degrade it. Of course, my solution is solution of problem, when you need to have separate grounds of PC and device. If you don’t have them separate due to device’s class, then you simply have no problem and will connect them directly. Now the series resistor 3R3 on the power. Standardly you need inside 3.3V supply but USB have 5V. The budget is intended mainly for cable wire resistance, so be carful and consult the standard for the maximum allowed cable wire resistance. My solution solves an present problem for lower supply current needs (case of my device). If you need full 500 mA, then you simply choose another solution, there is really no problem : ))
     
  4. elix-cz

    New Member

    Jul 28, 2015
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    PS: "My solution solves an present problem for lower supply current needs ..." Was thought a problem of dumping the resonator created by USB cable's power wire inductance and blocking capacitors. At PC USB power output may be a capacitor too, today it's long time before and I dont know what says the standard exacktly about this.

    The practice showed, that the biggest problem is to find quality USB cable. I tried different certified USB cable but their connection of cable shield to connector shield was done totally bad and useless for EMC immunity and reliability. At the and we used cables from Phoenix Contact.
     
  5. leon albert

    New Member

    Sep 17, 2016
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    It might be the case but no much often!
     
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