Where to connect shield on the PCB

[strahd_von_zarovich]

Hi everyone,
I have a system with several PCBs in it.There is an power distribution board in the system and I don't want noise coupling to my power lines. I am planing to use shield around wires. However I am not sure where and how to connect shield on the PCB. I have 2 gnd connections for every connector. So I am planing to use them like this, correct me If I wrong?

- I will use one of the ground connections for shieding and I'll connect it to the ground on only power distribution PCB, I won't connect it to the other PCB to prevent ground loops.
- I guess I don't have to use ferrite beads(like in the ethernet connections) to connect shield connection to ground since the wires carry only power ?
- My system has a metal enclosure, so the shield shouldn't touch this enclosure, right? I will use heat shrink tubes to isolate them.

Thanks in advance and forgive me If I am talking gibberish.

Strahd

 

[KlausST]

Hi,

It's not clear to me what "noise" you are talking about.
* common mode
* differential mode
* current
* voltage
* what frequency range
* what is the source of the noise
* what are your power lines' voltage signals? AC, what frequency, what waveform... or DC?

Without additional informations ... I doubt that shilelding power lines within a case makes sense at all.

Klaus

 

[c_mitra]

Thanks in advance and forgive me If I am talking gibberish.

You have made the point very clearly.

The subject of grounding is more of an art than science.

My suggestions:

Do not connect the shield at both ends;
If you are planning to carry delicate signals, please connect the shield(s) at the box end to the box ground;
Connect the shield on the receiver end (signal or power, as the case may be) where they are being consumed;
Ferrite beads can also absorb noise from power lines, particularly HF noise. Please use ferrite beads liberally (just like decoupling caps).

Good luck!

 

[strahd_von_zarovich]

Hi,

It's not clear to me what "noise" you are talking about.
* common mode
* differential mode
* current
* voltage
* what frequency range
* what is the source of the noise
* what are your power lines' voltage signals? AC, what frequency, what waveform... or DC?

Without additional informations ... I doubt that shilelding power lines within a case makes sense at all.

Klaus

Hi,
There are electronic modules like FPGA board, RF amplifier board, optic switch driver(it switches 400V at 10kHz), ADC/DAC board(Giga-Hz speed DACs and Mega-Hz speed ADCs), highly sensitive detector board(with 150dB gain) in my system. So I really don't know what to expect. All of the modules' power is provided by power distribution board. I tried to isolate their power lines by using filters and common chokes on power distribution board. There is a AC/DC power unit from Meanwell as a main supply of the system and output of the AC/DC is connected to the power distrbition board. Power distribution board only provides DC to the modules. I tried my best to isolate modules from each other by using metal enclosures for each of them. Modules' cases are connected to the bigger case which they are in.

My main concern is coupling between these modules. For example, any noise coupling to the DC power line of the detector module degrades my system performance since it has ~150dB gain. I tried my best to filter the noise(like using ferrites, caps, RC filters, using ultralow linear regulators like TPS7A4700).

 

[KlausST]

Hi,

No replies...

Your last post wasn't very informative... for me rather confusing.
The requested infornations are still missing - mostly
The given values sometimes don't relate to the problem (150 dB), or are incomplete ("optic switch driver(it switches 400V at 10kHz" what is this, what current, where do the signals go to?)
Now there seem to be many cases...
Is there no external wiring? Where do the DAC signals go to? Where do the ADC signals come from? Can they induce noise, ir create ground loops, or contain ESD pulses?

A saying is: a picture says more than a lot of words. This is especially true for technical descriptions ... and in a forum with people from all over the world...

It shouldn't be too difficult nowadays to take some photos, or give any value for current...
In short: some objective informations we can validate and discuss about.

Mind: You don't need to do this for us, you do it for yourself.

Klaus