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Why Shielding lines are connected to VSS not to VDD

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guard ring shielding+adc

kumarans said:
Shielding is to avoid cross talk so to safe gaurd the victim the victim is shielded .
Click to expand...

The topic is whether shielding lines should be connected to VSS or VDD. I guess everybody who are discussing this topic knows what is shielding and uses of it.
Please add valuable and relevant comments to the topic so that the visitor to the topic is not disappointed.

Regards,
Sandeep
 
Hello All,

In my opinion , let us say the net1 needs to shield. let us go to basics- usually we guide any unwanted signal to the ground not to the power. so we if we want to to shield the net1 we will place grounds nets, it will add a decoupling cap which is connected to ground. i forgot what we say the name of cap generated by neighbor net.
Contrary to it, if we shield the net by VDD, it will by pass only the disturbance from the VDD to the net itself by the cap formed by the vdd net and net1.

As we know charge flow from more charge to less charge, sheilding by ground is adopted over sheild by power.

I think , i didnt put in good words, can some one reshuffle my words to make it more clear..

thank you..
 
Generally speaking, VSS is clean and less noisy than VDD.
 

I wouldn't say that VSS is in general cleaner than VDD
Usually you have as many nfets switching as you have pfets switching, plus
some coupling between vdd and vss (cause you want to maintain vdd-vss
constant over time) -> compareable noiselevels for vdd and vss.

After all, when it doubt simulate. Make a model, including powerwiring,
Add switching noise, add coupling, make it as accurate as possible.
And don't forget the third and the forth variant, no shielding but more
spacing, and shielding with floating wires. As long as you not have
to deal with RF conditions this also might be worth a look

Best Regards

Andi
 

According to me .the shielding is done to a particular net with the signal that it is referenced to .Most of the nets are referenced to VSS .so shielding is done with VSS.
 

It probably makes little difference for noise reduction if the shields are connected to VDD or VSS. Usually there is more attention paid to making strong ground connections and therefore it will be a lower impedance, but that is not guaranteed.
 

I think Kapilsn is most correct; signals are usually referenced to VSS; this is why VSS is perceived as most "quiet" (since it is the reference to which everything is measured, it is by definition the cleanest). For AC signals, VDD may be regarded equivalent to VSS only if the decoupling is good enough (which generally is not the case). So I think it is mostly a matter of considering what the reference is from a system's point of view and not so much anything else.
 

* only by safety

* for not to convey power supply outside of the cables

FFFFFF
 

I think both VDD and VSS is ok, both VDD and VSS are low impedence node, so the noise voltage will be absorbed
 

Critical signals which are active on rising edge of reference are shielded with VDD
If the signal is active on the falling edge of reference then GND shield is used
 

In my mind there are times to use Vdd and times to use Vss.

Anyways, I would like to comment on the idea of using a floating wire as a shield mentioned by baenisch. This would be counter productive.

Floating metal between two traces increases the capacitance between two traces. See the derivation if you have doubts...

stefannm
 

Dear friends,
shielding lines are should we connected to the VSS only, not to VDD...
becos of shielding s are obviously used to reduce the signal noise. if we don't use the shielding in data line (critical)..if there is no metal across the data line (critical)..then no problem...if any of the metal nearer to the data line..due to the parasitic cap., we mayn't get the correct output...so,to avoid that we r going for shielding...
so, that shielding metal will protect out data line and whatever the charges occur in that metal it will be discharge it to ground...so we should connect it to ground......

for example..here with i have attached the image of double shielding....

this is my 1st post....
if anything is wrong reply me....


 

kapilsn said:
According to me .the shielding is done to a particular net with the signal that it is referenced to .Most of the nets are referenced to VSS .so shielding is done with VSS.
Click to expand...

you are right. the shield should be connected to the line (VSS or VDD) where the signal is referenced to.
 

Guys. everything is ok. You right that VSS is clearer then VDD. The one thing that I can ask of you is that during your chip design you have also VDDs which are generated from voltage regulators and so on, what if you'll connect your shield to that VDD does this shielding OK ?. Well one thing generaly you VSS power grid is more powerful then VDD that's why it's more sustainable to noise.
 

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