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I am designing a PCB for an image sensor. The manufacturer provided a design guide in which they recommend using multiple split ground planes.

One ground plane is for the analog supplies, but there are two split digital ground planes: one for the high speed serial data interface (bandwidths to 4GHz) and one for the rest of the (slower) digital interface signals (bandwidths up to the 10s of MHz).

I think I can understand the necessity of the split analog ground, however why would you split digital grounds? Shouldn't return currents be confined underneath the traces at such high frequencies? Is this to try to keep away the return currents of high speed interface (SLVS differential signals) from the interface signals, or the other way around: keeping the return currents of the slower single ended signals away from the differential interface?

Can I connect the two digital grounds a bit further away (before decoupling capacitors)?

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  • \$\begingroup\$ You should probably add a picture showing the planes and the signals. Maybe also add a link to the datasheet or design guide you are referencing. Modify your question. Don't reply in the comment section. \$\endgroup\$ – mkeith Feb 14 at 7:32
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    \$\begingroup\$ There are a few occasions I might split planes, but those are very rare indeed. As noted, a link to the datasheet / design guide would be useful. \$\endgroup\$ – Peter Smith Feb 14 at 11:26
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    \$\begingroup\$ One more vote for the datasheet, but I agree with Peter Smith that splitting ground planes often causes more problems than it solves. If it's possible to use a single ground plane and isolate analog from digital through careful routing, that would be my preferred approach. Note that this opinion is rising in popularity, but is far from being universally accepted, so it's worth listening to dissenting opinions before you decide. (Also because we're just commenting on general principles, since we're currently unaware of the particular part and requirements you're designing to.) \$\endgroup\$ – Cristobol Polychronopolis Feb 14 at 14:00
  • \$\begingroup\$ I guess I will chime in also. The best way to avoid cross-talk is to keep vulnerable signals (victims) far away from agressors such as high speed digital signals. Obviously if they all originate from the same IC, there is only so much you can do. But when you plan the board component placement, you can make sure that the signals do not cross each other and do not run along side each other. If you do all that, you probably don't need a split plane. And if you don't do all that, a split plane won't help you. It may make it worse. \$\endgroup\$ – mkeith Feb 15 at 2:05
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High speed digital signals are actually a form of analog signal, and are often spice modeled as such. Possibly without including a large budget for digital (slow period, but fast edge rate) ground plane or ground return noise. Thus a layout that corresponds to the (simplified) model.

Any cross connections between the two ground planes should not allow the two sets of AC loop currents to share a potential voltage drop.

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