Common Ground Plane and Power Planes Effects on PCB

[muthukumar_ece2004]

Hi,
All of us recommend that to use Power and Ground Plane. Fine, But why not the Ground plane split into AGND and DGND (Analog & Digital)?

Since combining of both ground should affect the circuit operation due to EMI noise. So how do we select the Ground plane? Why not use separate AGND & DGND Planes?

 

[FvM]

The general problem with separate GND planes is where to connect them. You'll notice, that data sheets of mixed signal devices, e.g. ADC, DAC, µPs with analog functions usually claim to get the separate GNDs connected directly at their respective pins. It's a clever suggestion for trivial evaluation boards, that utilize only one mixed signal chip.

Separate grounds involve voltage differences between both planes and differential currents injected between them. Both tend to make the board act as an antenna.

 

[muthukumar_ece2004]

oh...ok. So GND plane should not be related to anything such as Analog and Digital.

But i think, if we rout any one GND line instead of plane, we have to take care of both Analog and Digital, is in it?

 

[FvM]

Your initial question has been referring to "EMI noise". In this relation, ground traces of some lenth bring up a risk of sending or receiving interferences. There are however different coupling mechanisms. Resistive or inductive voltage drop will be the dominant one in low and medium frequency range. Keeping separate ground traces for interfering and sensitive signals is a possible means to reduce interferences. Even seperate ground planes can be reasonable in some cases. In my opinion, they lose their effect at high frequencies, however.