I was reading up on this and lots of papers say to make cuts. I understand why, but I am confused at the diagrams they have for implementation.

For example...

http://www.hottconsultants.com/techtips/split-gnd-plane.html

The diagrams show the split, but it connects the plane under the chip. However, I was envisioning just putting cuts around 3 sides and partially on the fourth and putting the whole ADC in there. Would that still suffice? I am kind of confused because if I do connect the plane under the chip, which side goes on the digital and which goes on the analog? Also, would it also make sense to put any op amp buffers in that cut plane with the ADC?

 

Analog circuits are noisy and can interfere with digital circuits. Keeping separate grounds (returns) for analog and digital is necessary to limit the noisy from the analog side of your circuits.

 

Analog circuits are noisy and can interfere with digital circuits. Keeping separate grounds (returns) for analog and digital is necessary to limit the noisy from the analog side of your circuits.

I beg your pardon but digital signals are noisy and they interfere with analog signals. Thus spoken from an analog guy.

 

In an ideal world when bringing together analog signals and digital signals it would be desirable to have two different ground planes, one for the analog and one for the digital signals. They would both connect together at the grounds pins of the A/D chip.

Edit: my anti-virus will not let me see your link.

 

The idea is that, since an ADC has both digital and analog sections you put the digital section over the digital plane and the analog section over the analog plane with a small connection between the split in the planes underneath the chip (this connection should ideally be the single point ground connection between the analog and digital supplies). Any other analog circuitry, such as op amps, goes completely over the analog plane, and digital circuitry goes completely over the digital plane.

To further minimize high frequency noise between the analog and digital planes connect them together through a surface-mount ferrite bead.

I once worked on a board that had excessive A/D output noise (in the digital bits), which I had to locate. I determined that it was due to a layout error where some of the digital circuit grounds were incorrectly connected to the analog ground. The board had to be re-laid out to correct the problem (which it did).