Connecting digital ground and chassis ground

[FreshmanNewbie]

I have an ethernet connector whose chassis or shield ground that I need to connect it to the digital ground. I read that it is a practice that is followed in many designs.

My questions:

1. Why should they be connected together? Any reasons? What would happen if they are not connected?
2. Also, in some designs 100nF and 50V capacitors are used to connect both the grounds. In some cases, I see 330ohm or 0ohm resistors? Whereas, in some designs, I see both Resistor and capacitor are used in parallel to connect both the grounds. Can someone tell me the pros and cons of each connection to arrive at a good circuit connection.

Thank you.

 

[KlausST]

Hi,

several reasons:
* to suppress common mode HF noise between communication partners.
* to avoid high voltage (static charge) to build up between communicaton partners.

But:
You should avoid to (possibly) create GND loops.

If you hard connect both GNDs:
* there is the possiblility for HF and LF down to DC GND loops

If you use a (series) capacitor:
* you avoid DC and LF GND loop, but still HF loops are possible.
* there is no suppression of static charge.

Thus use a capacitor in series with a low ohmic resistor.
* the resistor value needs to be high enough to lower the Q of a resonance, but low enough to still suppress common mode noise. (maybe 10R ... 100R)
* still there is no suppression of static charge.

Add a high ohmic resistor in parallel to the RC combination.
* This resistor in the MegaOhms just should suppress static charge.
(This is what I usually do)

But in detail it depends on both devices and the environment (noise, static charge...), maybe more..

Klaus

 

[FreshmanNewbie]

Hi,

several reasons:
* to suppress common mode HF noise between communication partners.
* to avoid high voltage (static charge) to build up between communicaton partners.

But:
You should avoid to (possibly) create GND loops.

If you hard connect both GNDs:
* there is the possiblility for HF and LF down to DC GND loops

If you use a (series) capacitor:
* you avoid DC and LF GND loop, but still HF loops are possible.
* there is no suppression of static charge.

Thus use a capacitor in series with a low ohmic resistor.
* the resistor value needs to be high enough to lower the Q of a resonance, but low enough to still suppress common mode noise. (maybe 10R ... 100R)
* still there is no suppression of static charge.

Add a high ohmic resistor in parallel to the RC combination.
* This resistor in the MegaOhms just should suppress static charge.
(This is what I usually do)

But in detail it depends on both devices and the environment (noise, static charge...), maybe more..

Klaus

Thank you. Can you please explain the below points a little more:

1. Can you share an image on GND loops?

2. What is the meaning of HF and LF down to DC GND Loops?

 

[KlausST]

Hi,

Please do an internet search for GND loops.

HF = high frequency
LF = low frequency
DC = no frequency at all, 0Hz

Klaus

 

[danadakk]

https://www.anixter.com/content/dam/Suppliers/Panduit/NI-IN-CPAG18-ENG.pdf

Search the cable guys web sites for apo notes on the topic, like Amphenol for example.


Regards, Dana.