Sheilding connection for connectors
bremenpl , 03-20-2019, 07:20 AM
Hello Robert,
Recently I have started watching your Advanced PCB Layout Course, I am at first lesson now. I find it that you give many useful side tips there. I have a question regarding the connectors casing connections. I see that in your design you connect all connectors casings (ie. HDMI or USB) directly to ground. In the video you explain that some connectors need to touch the device case for better pulses discharging path (ESD), that seems reasonable. In some devices however, I find it that the (USB especially) case is connected to the GND through a 10M resistor + parallel 2n2 capacitor for noise filtration.
Do you ever use this kind of connection? For example (image).
Source: https://forum.allaboutcircuits.com/t...-or-not.58811/
Is it a matter, that the "host" always grounds the shield and the "device" doesn't ground it, as we don't want to have both side termination? But if yes, is it an official rule?
Looking forward to your answer.
Lukasz
Recently I have started watching your Advanced PCB Layout Course, I am at first lesson now. I find it that you give many useful side tips there. I have a question regarding the connectors casing connections. I see that in your design you connect all connectors casings (ie. HDMI or USB) directly to ground. In the video you explain that some connectors need to touch the device case for better pulses discharging path (ESD), that seems reasonable. In some devices however, I find it that the (USB especially) case is connected to the GND through a 10M resistor + parallel 2n2 capacitor for noise filtration.
Do you ever use this kind of connection? For example (image).
Source: https://forum.allaboutcircuits.com/t...-or-not.58811/
Is it a matter, that the "host" always grounds the shield and the "device" doesn't ground it, as we don't want to have both side termination? But if yes, is it an official rule?
Looking forward to your answer.
Lukasz
robertferanec , 03-21-2019, 08:02 AM
I am not sure if USB specification defines USB connector shield connection. When I think, the most USB devices are not powered from Mains Electric Power (they are usually powered from the USB connector itself, they may use ungrounded power adapter or they may be battery powered devices). Some may be connected to Mains (devices using grounded power adapter, or for example UPS Backup Power supply can be an USB device and connected to Mains) .. and I am not sure how the shield is connected there.
Problem can be, if both, host and also device are connected to different mains power sockets and they are also connected through USB cable which has grounded shield - in this case, because there is sometimes potential difference between the two power sockets, currents start flowing through your devices and it can cause some noise on signals (sometimes you can even hear sound coming from your devices). And to prevent this situation (eliminate currents flowing between devices) you may want to use the circuit from the picture you attached (or something similar).
For motherboards I normally see shields connected directly to GND. I am not sure how USB devices connected to mains handle this situation - I have not designed anything like that ... so ... I guess, my answer did not really help.
Problem can be, if both, host and also device are connected to different mains power sockets and they are also connected through USB cable which has grounded shield - in this case, because there is sometimes potential difference between the two power sockets, currents start flowing through your devices and it can cause some noise on signals (sometimes you can even hear sound coming from your devices). And to prevent this situation (eliminate currents flowing between devices) you may want to use the circuit from the picture you attached (or something similar).
For motherboards I normally see shields connected directly to GND. I am not sure how USB devices connected to mains handle this situation - I have not designed anything like that ... so ... I guess, my answer did not really help.
Comments:
bremenpl, 03-21-2019, 08:44 AM
Hello Robert, thank you for answer. It is helpfull in a way that, it makes me believe that grounding the connectors on the host side is somewhat ok. This approach however seema only right for USB. In case of ethernet, there is no host and device, so probably both ends are grounded. There arent many ethernet cables with 1 side shielded connector only.
robertferanec , 03-21-2019, 08:58 AM
In case of ethernet, there is no host and device, so probably both ends are grounded. There arent many ethernet cables with 1 side shielded connector only.
Comments:
bremenpl, 03-21-2019, 09:24 AM
I gues its ground to gtound anyways
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