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Recently I've taken over a project which requires low noise but the design given to myself does not meet the requirements. I noticed on the PCB design It has a single Power Plane with 2 separate voltages (+5v & -5V).

Should I seperate these power sections into 2 separate planes to reduce potential for noise between the 2 voltage section in the single plane?

Single Power Plane - 2 Voltages

From my research it seems bad for noise to have a ground plane to have 2 separate sections (Analogue & Digital GND) as there can be voltage difference between the sections. People recommend connecting these sections together to reduce these noise issues. Obviously in this circumstance you can't connect these 2 sections, so that's why I thought separate planes might help.

Any and all help will be greatly appreciated. Thank you in advance!

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  • \$\begingroup\$ Which requirements are you talking about? \$\endgroup\$ – Anonymous Nov 1 '18 at 14:04
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    \$\begingroup\$ Are you driving a motor? Are you attempting to measure 1uV strain-gauge changes, at 1,000 Revolution Per Second axles rotation, to control torque? \$\endgroup\$ – analogsystemsrf Nov 1 '18 at 14:11
  • \$\begingroup\$ Requirement are just to have as low noise as possible. As it uses low noise amps. It appears the noise level is greater than the expected so I'm trying to improve upon the design and reduce any possible source of noise. I believe this is possibly a source of noise since there is a potential difference between the 2 planes. \$\endgroup\$ – SgtSafety Nov 1 '18 at 15:09
  • \$\begingroup\$ You can connect the two sections as far as AC currents are concerned. Just use a capacitor. Not that I would say you want to do that without knowing a lot more about your design. \$\endgroup\$ – The Photon Nov 1 '18 at 15:21
  • \$\begingroup\$ That is half my challenge, they haven't told me exact design requirement and won't get me the information. Was hoping this might be a simple design rule where you wouldn't put two different voltages on a single plane. I'll just have to keep researching. \$\endgroup\$ – SgtSafety Nov 1 '18 at 15:42
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Yes and No.

The problem with separate ground plane, it can become very complex to understand the current flow, and most of the time it is better to have a single ground plane.

Separate plane can be useful if you have filtering of the power and a lot of digital going on, but a lot of care needs to be taken.

The planes shall connect underneath the IC that has analog / digital, and you shouldn't have any trace going between the plane, at any layer. If you need trace going through from analog to digital plane, they should be above (or below) the connection between the two ground planes.

enter image description here

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  • \$\begingroup\$ Hi, Thanks for replying. I might be reading your reply completely wrong but I was discussing having separate power planes (+5V and -5V) as my the current design has that on a single power plane with a gap between both. Again sorry if I've misunderstood your reply. \$\endgroup\$ – SgtSafety Nov 1 '18 at 14:37
  • \$\begingroup\$ Can you add on the screenshot which plane is what ? is there a ground plane on other layers? \$\endgroup\$ – Damien Nov 2 '18 at 11:45
  • \$\begingroup\$ The centre plane of the image is -5V & the outside is +5V. \$\endgroup\$ – SgtSafety Nov 5 '18 at 8:59

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