# Electrical shielding newbie question

Discussion in 'Electronic Design' started by Yvan, Jul 7, 2005.

1. ### YvanGuest

Hi all,

I have a question about electrical shielding.

I just read the book "grounding and shielding techniques" and there's
one thing i don't understand.

The whole point of electrical shielding is that you provide an easy
path for unwanted currents to go to ground, instead of going through
your circuits. Is this right? In that case, it suggests to ground the
shield where the voltage source is only. I'm trying to figure out the
problem for an analog sensor connected externally to our device. It
would share the signal and ground conductors withthe data acquisition
system.

Our enclosure is in plastic. If the shield is connected to the ground
of the sensor and then to the enclosure. How can current pass through
there? I don't see the point of a shield if the enclosure is in
plastic (open-circuit?). In that case, wouldn't current go through the
common ground conductor instead and create ground noise? How should I
connect the shield then?

Thank you,

Yvan

2. ### John PopelishGuest

That is one purpose for shielding. Shielding deals with local
magnetic and electric fields and emission of radiated waves.
Sometimes you contain the source of noise with a shield, and sometimes
you are not in control of the source of the field, so you contain the
circuits subject to disturbance from fields. Sometimes you do both.
Changing electric fields cause displacement currents that connect the
object to the voltage source. Changing magnetic fields cause local
eddy currents in conducting loops. Either cause of current will cause
voltage drops in conductive paths.
The way I see it, the shield is an extension of the box that contains
the converter, so that it also contains the sensor and its wiring. It
should be separated from all wiring, except for a connection at one
point. In this case, I would pick a point very close to he converter.
That way, all currents that slosh around in the shield conductor in
response to electric or magnetic fields produce little voltage drop in
any of the signal conductors, including the signal common.

3. ### YvanGuest

I don't totally understand your last paragraph. In our case, our box
has signal conditioning for a 0-3V analog sensor input. We provide
with the ground. In other words, the A/D converter is in our plastic
box and the sensor is in the other one. If I understand well, you
would make one single connection of the shield with the ground, close
to the converter, in our box? Every book I have read mentions it has
to be close to the source of voltage, hence the sensor....

I know that you either shield from electric and/or magnetic
disturbances. In our case, I'm more concerned about electric
disturbances, as the signal frequencies are in the order of 1 kHz.
The sensor will have little disturbances from external sources as there
won't be transformers and such close by.

In that perpective, I don't understand how such a shield would protect
against electrical disturbances if I have a plastic box. How would it
change the electrical fields then? Where are currents flowing with and
without that shield? I wish I could draw what I see in my head...

Thank you,

Yvan

4. ### John PopelishGuest

What, electrically, do you mean by, "we provide with the ground"?
I generally connect the shield to the signal common at the point where
I care about the signal integrity the most. I would wrap that shield
around as much of the wiring and system as needed for it to be the
capacitor plate that receives change displacement form external
electric fields. If capacitive currents are dumped through the
shield, it will pass these currents to signal common at the connection
point, and that may bounce the potential at that point around (and all
the signals, with it). But if all signals and signal common share the
same noise at that point, the circuits that responds to the difference
between signal potential and signal common potential at that point all
share the same noise addition, the differences between them will show
little of that noise.
Another source of magnetic interference is power lines running
Feel free to email any sketch you may have to me. If I have any
useful comments, I will post them along with your graphic on the
newsgroup alt.binaries.schematics.electronic, as well as by return email.

5. ### YvanGuest

Ok thanx, I will come back to you shoftly.

Yvan

6. ### YvanGuest

Ok thanx, I will come back to you shortly.

Yvan