What is the optimum distance between split power planes ?

pcb split planes distance between

Hello more experienced,
In a 4 layer board(a PCI card), I am using both 3.3V and 5V components in the same circuit. So I have got two power planes (3.3V and 5V)on the same layer.

layer 1:signals
layer 2:Ground Plane
layer 3ower Planes ( 3.3V and 5 V.)
layer 4:Signals.

And I have got eight(8) digital bus signals which should cross over that splitted plane.
I read that I should couple those planes with some 100nF capasitors. And the number of the capasitors is recommended half the number of the signals.So I used 4 capasitors from 3.3V to 5V for that 8 signals. Frequency of the bus is around 1Mhz. Now I extremely wonder what should be the optimum distance between the power planes so that my signals are not effected while crossing the gap. What is the relation between gap and signal wellness(integrity?)? Is it better to make big gap or small gap? how many mil or mm do you suggest for that gap? Thank you in advance.

Re: What is the optimum distance between split power planes

generally gap between plane split taken is 20 -30 mil
if gap is more then noise coupling between plane split is reduced ,
but more gap create problem to signals crossing over split .
so it should nither more or less

with regards

vij

20 mil Separation gap is better

Re: What is the optimum distance between split power planes

Hello baskan,
about this interesting sentences...
"I read that I should couple those planes with some 100nF capasitors. And the number of the capasitors is recommended half the number of the signals.So I used 4 capasitors from 3.3V to 5V for that 8 signals."
where do you have find this rule ? And where have you place the capacitors ?Near the
trace...near the loader... ???

best regards
mkbs

Re: What is the optimum distance between split power planes

The minimum preferred distance that is being conventionally followed for the digital power planes, keeping in mind about the Si effects and DFM is 20mil = 0.5mm. Generally, you cannot avoid the bus signals crossing between two split planes. 0.5mm is a very small trace length(< than critical length of FR4), for a 1Mhz bus, to cause cross talk and other coupling effects.

20Mil is good to go