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analayout
Joined: 20 Feb 2007
Posts: 94
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 16 Sep 2008 5:55 Why Shielding lines are connected to VSS not to VDD |
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Hi All,
Why Generally shielding lines are connected to VSS not to VDD ?
Regards,
Analayout.
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andrew_matiga
Joined: 21 Aug 2006
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| 16 Sep 2008 6:05 Why Shielding lines are connected to VSS not to VDD |
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hi analayout,
Shielding is for radiated emmisions and radiated immunity. For example, in board layout, you put shields between high speed clock and the data lines because the high speed clock lines might radiate emi noise to other data lines. The shield will just recieve these emi from the clock lines and dissipate them to ground(Vss).
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analayout
Joined: 20 Feb 2007
Posts: 94
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| 16 Sep 2008 16:14 Re: Why Shielding lines are connected to VSS not to VDD |
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Hi ,
Is emi shielding and electro static shielding are same ?
Wht i meant is shielding in high freequeny Ic layout eg:- shielding of Differential pair input in ADC layout.
Regards,
Analayout.
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andrew_matiga
Joined: 21 Aug 2006
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| 16 Sep 2008 17:02 Why Shielding lines are connected to VSS not to VDD |
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Hi analayout,
Im not sure if both shieldings are the same in IC design. But in board level, it is different.
I am not also familiar with ADC layouts, but as what ive mentioned on the previous note, shields should be connected to ground(Vss) becuase it needs to dissipate the noise to ground. It applies to all shieldings, it may be in the board level or IC level.
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JoannesPaulus
Joined: 19 Mar 2008
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| 16 Sep 2008 17:13 Why Shielding lines are connected to VSS not to VDD |
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| It does not really matter if you connect the shield to VDD or VSS, you just need to connect it to a low impedance net.
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umery2k75
Joined: 19 Apr 2006
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| 16 Sep 2008 22:52 Re: Why Shielding lines are connected to VSS not to VDD |
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| What shielding lines are like, like other similar copper track in the PCB?
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tlihu
Joined: 02 Jan 2002
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| 16 Sep 2008 23:14 Why Shielding lines are connected to VSS not to VDD |
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| Because VSS is usually more stable and stronger than VDD.
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umery2k75
Joined: 19 Apr 2006
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| 17 Sep 2008 10:10 Re: Why Shielding lines are connected to VSS not to VDD |
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| How could you say one is more stronger to other? On what basic. Both are voltages in the light of physics.
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k_90
Joined: 25 Jan 2006
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| 17 Sep 2008 22:04 Re: Why Shielding lines are connected to VSS not to VDD |
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net which require sheilding must be tied to ground as the vdd is normally the noisy signal due to switching.
Look up the alan hastings book.
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sandeep_torgal
Joined: 02 Jan 2008
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| 17 Sep 2008 22:16 Re: Why Shielding lines are connected to VSS not to VDD |
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| andrew_matiga wrote: |
hi analayout,
Shielding is for radiated emmisions and radiated immunity. For example, in board layout, you put shields between high speed clock and the data lines because the high speed clock lines might radiate emi noise to other data lines. The shield will just recieve these emi from the clock lines and dissipate them to ground(Vss). |
Shielding is done in a different way as far as analog layout is concerned. It is not done for the clock lines but we shield the data lines. It will be similar like we insulate a electrical wire.
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Added after 2 minutes:
| JoannesPaulus wrote: |
| It does not really matter if you connect the shield to VDD or VSS, you just need to connect it to a low impedance net. |
It does matter if the shielding is done to VSS or VDD.
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JoannesPaulus
Joined: 19 Mar 2008
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| 17 Sep 2008 22:32 Why Shielding lines are connected to VSS not to VDD |
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| sandeep_torgal, I believe (and I have measurements to prove it) that in principle it doesn't matter if you use VDD or VSS as your shield. As I said, it just as to be a low impedance net. In a single supply system, the current taken from VDD will also flow on VSS. The difference is that VSS might have more bond wires and has surely a nice ground plane off-chip.
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sandeep_torgal
Joined: 02 Jan 2008
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| 18 Sep 2008 3:29 Why Shielding lines are connected to VSS not to VDD |
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We should tie the shield to VDD/VSS depending wheteher the current is source or sink. This will inturn help to reduce the cap between the data line to the shielding lines.
Correct me if am wrong or if there are other opinions.
Regards,
Sandeep
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JoannesPaulus
Joined: 19 Mar 2008
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| 18 Sep 2008 19:11 Why Shielding lines are connected to VSS not to VDD |
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I believe you are confusing shielding and decoupling. I agree with you that if you have, for instance, the gate of a pmos in a current mirror you might want to decouple it with respect to VDD and the gate of an nmos wrt VSS but this is not shielding.
What if you want to "protect" the input signal of your ADC from the clock? Is it sinking or sourcing current? Neither.
In this case you want to have a line that runs parallel to your input signal - between your input signal and the clock - and connect them to either VDD or VSS. Now you are shielding the input signal.
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sandeep_torgal
Joined: 02 Jan 2008
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| 18 Sep 2008 21:53 Re: Why Shielding lines are connected to VSS not to VDD |
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There is no confusion as to the shielding and de-coupling.
Let me know if you ever shield clock lines ? Why not? Its due to the coupling cap because of shielding.
For protecting the input of your ADC from clock, we sheild the input and tie the shield to ground. Ground is more preferred as we may not route the VDD powerlines all over.
Regards,
Sandeep
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JoannesPaulus
Joined: 19 Mar 2008
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| 18 Sep 2008 22:25 Why Shielding lines are connected to VSS not to VDD |
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I usually do not shield clock lines (they are the aggressors to my analog signals) for the exact reason you mention: the shield might load the clock unnecessarily - and therefore increase the power consumption. I only shield important analog signals (i.e. input signals, bias lines...).
I agree with you that VSS is probably easier to use than VDD.
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jecyhale
Joined: 19 Feb 2008
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| 24 Sep 2008 2:59 Re: Why Shielding lines are connected to VSS not to VDD |
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| The noise of VSS is the least in the chip.
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nozone
Joined: 16 Jun 2003
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| 04 Oct 2008 7:43 Why Shielding lines are connected to VSS not to VDD |
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if you use the ptap on a p-substrate, you must connect to VSS, if you use NWELL and ntap(which make the size bigger) as guard ring, it must be connected to VDD.
if it's metal, yeah, I agree that the VSS is uaually more stable and available.
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