Shield for differential clock

[ritchyv]

Hi,
I would like to create a shield around a differential clock on the metal2 (Virtuoso Layout IC6.1.5 - IBM 130nm). How do I do this ?

Thanks

 

[erikl]

Adjacent routes on metal2 + wide routes on metal1 & metal3, all connected to GND can make a good all-around-screen. But of course create massive cap losses.

I wouldn't advice it. If your differential clock has good symmetry - both regarding drive strength and fan-out - crossing signals receive opposite stray signals which well cancel out. Nearby sensible inputs could be screened individually.

 

[bgangur]

Hi,

You can use normal shielding and co-axial shielding depending upon the critical on those diff nets.

1. Normal shielding -- side by side
Metal2 is you conductor
Add M2 both side of the conductor with 1:2:1 spacing.

2. Co axial

Adding M3 on top sides with width to cover fully the conductor signals
Adding M2 on both sides of the conductor and also drop VIA3-2 to connect M3 and VIA1 to connect M2
Adding M1 on below sides with width to cover fully the conductor signals
Here also you can maintain the 1:2:1 spacing between the shield and conductor.

Thanks,
Basu