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I'm designing a circuit that uses an optocoupler to isolate a relatively noisy H bridge from the USB circuitry also present on the board. Clearly in this case I will have two grounds - what is the norm for dealing with these in terms of ground planes?

My initial thought is to have 2 ground planes, 1 on the USB side and 1 on the H Bridge side of the board (isolated from each other), but could this cause problems with capacitance if one of the grounds were to rise above the other?

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  • \$\begingroup\$ Do you have a separate supply for the H-Bridge? You can galvanically isolate the whole thing (separate power supplies, ground planes) and use the optocouplers to transmit control signals. \$\endgroup\$ – kva Jun 11 '17 at 13:31
  • \$\begingroup\$ @kva That's exactly what I ended up doing, and for my application it seemed to work fine - but my worry was that high frequency noise on one could emerge on the other via capacitive coupling. \$\endgroup\$ – berry120 Jun 11 '17 at 14:00
  • \$\begingroup\$ That shouldnt be a big issue if there's enough clearance between the planes. \$\endgroup\$ – kva Jun 11 '17 at 14:17
  • \$\begingroup\$ @kva Yup, I used a gap of a few cm eventually and decided that would be good enough. \$\endgroup\$ – berry120 Jun 11 '17 at 14:22
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Your concerns are justified, as two adjacent ground planes are just perfect for capacitive coupling of high-frequency noise from one to the other.

The standard approach is called a split plane. A split plane is just what it sounds like: your ground plane is divided into two separate areas which are not connected except for one point. This keeps the bridge noise (on one ground plane) separate from the signal processing (on the other plane). The disadvantage with this approach is the need for isolated IO. Your power connections, for instance, have to come in on two separate connectors, one for each plane. Likewise with signal connections. And, of course, you need to restrict the circuits to the area above the appropriate ground plane.

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    \$\begingroup\$ One could argue that the benefits of opto-coupling between split planes are lost when such planes are tied together (even if at just one point). \$\endgroup\$ – Michael Karas Sep 6 '15 at 14:46
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    \$\begingroup\$ @MichaelKaras I had come across split ground planes before but (as you suggest) was worried that linking them at a single point would nullify the benefits of opto-isolation. Is there something else that you'd suggest? I could of course do away with the ground plane entirely and just remove the stray copper, but didn't know if that would cause other issues. \$\endgroup\$ – berry120 Sep 6 '15 at 18:53

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