some questions on layout?

Hello, I am not sure about some rules of the layout and I am wondering are there any reasons for these rules.


1, The ground plate should be as large as possible.
It is also better for the voltage supply plate as large as possible, therefore there is a very large overlap area between the ground plate and the voltage supply plate.
-----so the votage supply on the whole chip will be stable, the current are distributed on the whole chip?

2, It is better for the signal line not to overlap with the other signals, and it is also better to avoid the signal line to overlap with the power supply plate???
Could signal line overlap with the ground plate? ??
-----to reduce the coupling between the signal line and the power supply?

1) Yes, the ground plate and the power plate should be big enough to decrease the impedence. The capacitance between the POWER and GROUND is welcome.

2) Of course, you should take care about the routing of the signal path. You have to avoid to cross noise signal, power or gorund.

Bye

1.First, the supply almost have noise. Adding the cap between gnd and supply, it can reduce the nosie infulence. For example, while the supply have a small nosie, the gnd will have the same if the cap is much big, so the supply-gnd=(supply+nosie)-(gnd+nosie).just i think.
2.While rout the signal line, you can rout it parrell the gnd. it can reduce the noise infulence because of the parasitc cap. If the signal line is very sensitive, you can shield it using two gnd lines arround it like a box. You can refer to you circuit designer.
The art of analog layout is a good book, you should get it. And the basic of mask layout is also a good book.

dabing said:

1.First, the supply almost have noise. Adding the cap between gnd and supply, it can reduce the nosie infulence. For example, while the supply have a small nosie, the gnd will have the same if the cap is much big, so the supply-gnd=(supply+nosie)-(gnd+nosie).just i think.
2.While rout the signal line, you can rout it parrell the gnd. it can reduce the noise infulence because of the parasitc cap. If the signal line is very sensitive, you can shield it using two gnd lines arround it like a box. You can refer to you circuit designer.
The art of analog layout is a good book, you should get it. And the basic of mask layout is also a good book.

Click to expand...

I guess there is a mistake here, Are you giving an expression for the supply noise?
supply-gnd=(supply+nosie)-(gnd+nosie)
If this is true, differential circuits wouldn't have noise at output and the problem of noise would be overcome.
Remember that when you are dealing with noise analysis, you assume additive noise model, so at the output of a differencial circuit you will have 'V1noise+V2noise'.

Well, for the original post:
1)
Climber73 post is right, Supply-gnd capacitance is wellcome, but Why? When you have high frequency variations in supply source, for example in mixed-signal desings, the supply-gnd capacitance will filter the high frequency variations making the voltage 'stable', the capacitance behaves as a short circuit.

2) If you overlap a signal with other signal you will have electromagnetic coupling, the amount of coupling will be given for geometric and electric characteristics. Remember that you will have capacitive and inductive effects. For this reason it is suggested don't overlap signal paths with voltage references, to avoid coupling. Finally, is better route signals with ground overlap, you will have a capacitor to groud, but you will shield the signal, i. e. you will protect the signal from coupling to others...


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The large plate of ground may console some IR drop

The large plate for power & the ground is used to avoing electomicgration. Electromigration means a surge current passing through the power supply line & finally damaging the power lines . To avoid this we genaraly use large metal widths.

The gnd and supply need to be wide to provide a low res path, because all the block is using the same supply and gnd, the high parasitic res may arise crosstalk between different blocks and worse the operation enviroment. The supply and gnd cap is welcome becaulse it may form a low path filter with parasitic res of the supply and gnd line,so maintain the voltage stablization.
Singal has a reference,so the sinal should be vary with the reference. and it mustn't be interfered by other signal. Customarily,the reference is gnd, so we offen use the gnd as a shield.

In case I am laying out a mixer, RF inductors and IF inductors use Metal 1, 2 connected by via 2,3 (I use pHEMT process). If I use Metal1,2 and via 2,3 routes (and grounded) in trying to seperate the RF and IF inductors, is that useful?