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Im currently building a signal generator and have a problem with placing multiple different ground planes which have different voltages.

The system is powered off of USB, so all my digital logic signals are referenced to the usb ground. I also have a virtual ground generated at half the voltage in order to power my analog circuitry. This means that all my digital signals are referenced to digital ground (0V) where as all my analog signals are referenced to virtual analog ground (2.5V). The analog signals consist of ~8MHz precision sine waves and their signal integrity takes priority.

Im developing this on a 2 layer board so I don't know if the bottom layer should be entirely digital ground, entirely analog ground, or a combination depending on the sub-system it's directly under.

Thanks!!

block diagram of power and sub-systems

Ive placed a simplified block level diagram of my solution, but let me know if you wish to know any finer details!

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  • \$\begingroup\$ You definitely want to keep digital anything ground or otherwise away from anything analog virtual ground or otherwise. So I would put the digital circuits in one area and analog circuits in another and use separate ground planes. The most important thing will probably be to connect the negative rail of the analog circuit at the same place where the usb ground enters and also put bypass caps at the same spot. If you connected the negative rail to some place far from the USB ground spot and didn't bypass, digital noise will bleed into the neg rail. \$\endgroup\$ – squarewav Feb 8 '16 at 22:59
  • \$\begingroup\$ @squarewav thanks a bunch, that was a lot of help! \$\endgroup\$ – Shaheen Moubedi Feb 9 '16 at 3:45
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Dedicate bottom plane to digital ground plane - that approach makes even better sense if you are using MCU or other digital or mixed signal IC's on PCB.

Signals referenced to virtual ground will be low frequency(~DC) so ground plane will add less benefit.

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  • \$\begingroup\$ Thanks for the feedback! Just wanted to add that the analog signals referenced to ground will be ~8MHz sine waves and I am prioritising their signal integrity. If my entire ground plane is digital ground, won't some of the digital signals couple on to my analog signals as noise? Or am I over thinking this? \$\endgroup\$ – Shaheen Moubedi Feb 8 '16 at 17:23
  • \$\begingroup\$ Do not use entire ground for digital. It could easily bleed in. Bip is not giving good advice. You are not overthinking. See my comment above. Correct grounding could make a huge difference in noise in this case. \$\endgroup\$ – squarewav Feb 8 '16 at 23:05

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