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What will happen if we do not connect shield lines to any net in analog layout ?

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RohithRaj

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Normally we connect the shield lines to VSS or VDD. but what will happen if we do not connect the shield line to any net ? - ie,what will happen if we keep shield line as a floating net ?

I know one reason --- floating shield will create additional capacitance to the victim net -- i want to know all the possible reasons
 

Hi,

What circuit or application are you talking about?
Give more details.

floating shield will create additional capacitance to the victim net
Click to expand...
Floating or not - the capacitance is the same.

Klaus
 
My doubt is related to general analog circuits (eg op-amps).
Why do we connect shield net to VSS/VDD generally ?
why don't we keep the shield net as a floating metal?
 

In fact, both options will work as shields, "grounded" to VDD/VSS/GND or just keeping floating, but as charges are induced in the conductor, an electric field will be radiated from that line. If you force that material to have a steady potential and if it is tied to a net with low impedance and virtually unlimited capacitance, it will be performing a more effective shielding than just putting a metal barrier between the agressor and the victim, which depending on the case, such railing could even turn things worse, spreading the induced field to more sensitive areas of the circuit (ie, high impedance nets).
 
A floating shield not connected to ground or Vcc will pickup interference and feed it into the signal wire that must be shielded.
At the amplifier, disconnect the shield on the cables from a magnetic phono cartridge and hear the loud hum caused by no shielding.
At the oscilloscope, disconnect the shield on the probe cable and see all the interference it picks up.
 
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