Question about power and gorund plane

power plane return current

I have separated my digital power & ground plane and analog power & ground plane by few inductors.

Do you think what I have done is right?

The value for those inductors should be how much?

Thank you for your any suggestions.

yes what you have done is right...
the values of the inductors depend on the frequency of noise u can allow on ur plane. what band u want to couple and not to couple decides ur inductor value. the inductor across the planes form a pi filter... if possible add capacitors to improve ur pi filtering... calculate accordingly....

Inductors in Analog & Ground plane is not required. Just a 0 ohm resistor is enough. The sole idea is to differentiate the two nets. You need a mechanism to connect these two planes when you are releasing gerber filed.

Having said above, you must logically divide board in analog & digital seccion & make sure you maintain correct gnd & power planes.

Regarding placing inductor in VCC plane. Indictors are widely use. You have to make careful judgement. Blindly using inductors i some cases can backfire you.

Good Luck

There are also great trains of discussion that say that splitting the ground planes is not a good idea.

That ensuring that analogue and digital sections ar kept apart but that they can share the same groundplane. That the signal follows it's return path underneath the signal track back to the source and it's best not to split the plane.

I.E. here:

centiago said:

Inductors in Analog & Ground plane is not required. Just a 0 ohm resistor is enough. The sole idea is to differentiate the two nets. You need a mechanism to connect these two planes when you are releasing gerber filed.

Having said above, you must logically divide board in analog & digital seccion & make sure you maintain correct gnd & power planes.

Regarding placing inductor in VCC plane. Indictors are widely use. You have to make careful judgement. Blindly using inductors i some cases can backfire you.

Good Luck

Click to expand...

nod

my view:
digital power plane -----Inductor(or ferrite bead)-----analog power plane
digital ground plane ----zero ohm resistor----analog ground plane

my sincere thanks to all of you.



Added after 7 minutes:

one more question?

The analog return current and digital return current are going to be mixed on ground plane?

Is this going to generate big noise to analog circuit?

Thanks.

the main concept of using a split plane is not to create different nets.... it is to avoid the coupling of the return paths of the analog and the digital sections. the only path between the planes is the bead connected in between... which filters the ac portion of the signal and it couples only the dc, which is what wanted. but it is necessary to take care that the digital signals are not routed over the analog planes and the rule is same analog signals. i think it helps...

I am confused with following:

the signal follows it's return path underneath the signal track back to the source and it's best not to split the plane.

Click to expand...

this means the return curent is going to take ground?

EDA_hg81 said:

I am confused with following:

the signal follows it's return path underneath the signal track back to the source and it's best not to split the plane.

Click to expand...

this means the return curent is going to take ground?

Click to expand...

Yes. return current going directly under signal trace in groundplane depend of electrostatic field and magnetic coupling, and this coupling going harder on higer freqency.

If you have slot or wide gap in groundplane under trace, force return current going longer way around slot, electromanetic and electrostatic field expand in area and volyme and begin working more as antenna and sending out electromagnetic wave - and you begin have trouble with electromagnetic noise and over hearing on other near going traces. (this slots give also differet characteric impedances on traces and make more or less reflection)

common gnd technice with small gnd traces using in old digital circurit is not good solution for intergity of signal trace and problem rises quickly in higer frecuency domain, depend of all signal traces will comminucate to return current in single GND-traces via magnetic and elecrostatic field (and using near trace as short cut if this way make easier for return current - and this way make overhearing), so groundplane without slot is easy way to make virtual return traces on every signaltraces and make minimum signal degration, ovehering and EMI-radiation.

Make ground-plane (and connect all gnd-point on this) as middle layer in old 2-layer digital design can easly decrase EMI-radiation between 20 - 40 dB and ground level is going much stabile and analog circurit (ex. A/D-converter) on card reciver lower level of digital noise.

hai centiago
what is the need of placing 0-ohm resistor between palnes????
if u place like that meance we r shorting both planes
so that board may not work properly

Hi centiago,


There is no need to use a zero ohm resistor ,if you use zero ohm resistor then the noises will couple from one split to another.Then why do people use ferrite beads between two splits in first place? If you say yes for Zero ohm resitor then please shed some light on the topic....


Regards

Ramesh