question about two power wires

[vadim888]

question about power wire

Hi all,


I have few questions about vdd and gnd wires placement.

Is it better to place vdd over gnd?

• f.e. m3 is gnd, m4 (top) is vdd.

What do you think about distance between analog vdd/gnd and digital vdd/gnd in typical cmos 1.8u process? How better to place analog and digit wires.

Thank you all,

Vadim.

 

[erikl]

Re: question about power wire

Is it better to place vdd over gnd?

• f.e. m3 is gnd, m4 (top) is vdd.

In pad rings the power lines usually are on same metal level (because the space beneath is used by I/O and ESD protection circuitry). But there's no reason why not to put them over each other, with the additional (desirable) effect to achieve some (tiny) parasitic capacitance between them.

What do you think about distance between analog vdd/gnd and digital vdd/gnd in typical cmos 1.8u process?

Just use the width-dependent design rules.

How better to place analog and digit wires.

Depends on your requirements: for shielding analog signals use min. spacing. Use wider spacing to keep parasitic capacitance low, if necessary.