Does Differential pair required ground plane ?

[Kabaleeswaran]

Hi Team,
i am new to Design.In My design, there is low speed Differential pair and High speed Differential pair available. Even i maintained 2W spacing between Diff pair to Diff pair. Some section ground trace running in between high speed differential pair to avoid the cross talk. low speed signal there is not need of ground plane for Diff pair.

the requirement are,
1.cross talk able to meet to provide proper spacing between differential pair 2W spacing .
2.EMI able to avoid ground trace between each differential pair.

So what is the major role for ground plane in high speed differential signal ?. or does differential pair need ground plane ? because one signal is reference for another signal .can you explain short answer for this query.( i red so many document. little bit confusion. ).



can any one explain is it possible to meet the requirement with out ground plane ? With out ground plane is not able to control the cross talk , EMI issue and other.

[Paul van Avesaath]

not 100% sure.. but technically you do not need the GND plane for a diffpair the GND plane is there for impedance. so how did you calculate the impedance without a reference plane?

for crosstalk i would go as wide as you can between the .. so 3 or 4 times the width if you can manage..
i do see some wierd openings in the GND traces you draw between the diffpairs (but that could also be a glitch in the screenshot)

in general you need a GND plane for the return current so i would highly advice adding it.. it will be better for the whole design.

[robertferanec]

Some section ground trace running in between high speed differential pair to avoid the cross talk.

- did you include these GND tracks between pairs when you calculated impedance? also, I believe, if you use this kind of technique, you may need to place VIAs in more places, not just at the beginning and end.

1.cross talk able to meet to provide proper spacing between differential pair 2W spacing .

- bring the sold ground plane under the diff pairs close to them, that will help to eliminate crosstalk. Also, please keep in mind, when you see a recommendation such 2W these are usually done based on specific impedance. this video may help: https://youtu.be/Bu4MirknEvo

So what is the major role for ground plane in high speed differential signal ?. or does differential pair need ground plane ? because one signal is reference for another signal .can you explain short answer for this query.( i red so many document. little bit confusion. ).

- I have the same question what Paul van Avesaath asked: How did you calculate impedance if you don't have a reference plane?

Also, don't forget. In some cases / interfaces, differential pairs are just two single ended tracks with opposite signals running through them => e.g. even when routing these differential pairs, each track in pair needs to meet specific single ended requirements, for example they have to be routed by 50OHM impedance.

[Kabaleeswaran]

Hi
paul and robert Thanks for your update. I agree with your point. I need one more clarification,
L1-Top
L2 -Ground plane ( number of signals are more so taken more signals in ground plane)
L3 -pwr
l4-Bottom

there is ground plane as your suggestion diff pair required ground plane to meet impedance so instead of ground plane shall i route ground trace in adjacent to diff pair which help to meet impedance ? due to space , density and cost and other .please guide me.

[robertferanec]

Did you try to check the price difference between 2 layer and 4 layer PCBs? These days 4 layer PCB is not expensive, but are much better. Solid GND has many advantages and it will make your PCB much more reliable and stable. Also solid GND saves a lot of space (you don't have to route GND tracks).

PS: routing gnd tracks is not as good as solid gnd plane. also you would need to calculate it differently, have a look at coplanar calculation.