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what is the effect of floating net in layout

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gsarith

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Hi All,
In my layout there is a long running metal which is not connected to any active nets (floating). Does this can cause any effects on the adjacent signal nets on the same/different layer?. Since the net is not connected I am unable to think of any capacitance related issue but I feel that due to the switching in the near by nets the floating net can accumulate charges and can be on a higher potential w.r.t ground, can any one please explain me the what can be the actual problem in this case. Also a layout guideline is to avoid long running nets even though they are floating, if possible please explain the reason for this guideline also
 

Pls have a post-layout simulation if you worry about it.
 
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    vpabba

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leo_o2 said:
Pls have a post-layout simulation if you worry about it.
Click to expand...

hi, thanks for reply, I am basically a layout designer and have no access to simulation tools. It will be helpful if you can explain me theoretically what will be the effect of the above said condition.

Regards,
Sarith
 

A long net can be split briefly changing metal layer. I do not think it is a problem wrt charge accumulation but it might capacitively couple two (or more) sensitive nets that where far away.
 
if the floating net between very aggressive net & very sensitive net.... then the floating net act as a intermediate net between nets and sensitive net may be affected just like as you said by charge accumalation & capcitance... so its always better to clamp this type net to vdd or ground
 
what i' m thinking is some emf has formed in that floating net by the nearest potential net .so it should form coupling capacitance between those nets ..
 

As others suggest, it might lead to coupling noise by coupling capacitance.

gsarith said:
hi, thanks for reply, I am basically a layout designer and have no access to simulation tools. It will be helpful if you can explain me theoretically what will be the effect of the above said condition.

Regards,
Sarith
Click to expand...
 

It gives coupling noise by coupling capacitance. So better you connect it to any power line nearby. Generally floating metals are not preffered in layouts.
 
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