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When need I seperate digital ground and analog ground?

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ifarmer

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phy analog ground

To keep digital noise from interfering with analog signals, we may seperate analog ground from digital one, and then connect them at one point or via ferrat beans.
But this way will increase ground impedence. So recently, I always combined these two grounds, especially in high speed design.

So when we need seperate ground plane?Are there any advice?
thanks[/b]
 

ifarmer said:
... But this way will increase ground impedence. So recently, I always combined these two grounds, especially in high speed design.

Don't think the ferrite bead as a inductance, it's a very lossy impedance at some frecuency band (this depends on core, turns etc). So what you get is insertion loss, i.e. HF energy doesn't reach your common ground nevertheless you still see a good ground (not one with a higher inductive impedance) from your noisy HF circuit.
 

The usual method is to use two separate ground and power planes and connect the grounds together where the digital and analog signals meet such as the ADC or DAC. The power planes should be connected together at the power supply output.
 

When I design on PLL or PHY of Ethernet, I always treated AGND same as DGND, and lay a small AVCC in signal plane.

Is this method just easy to practise, or is also good for analog part?

In fact, I changed an interface board of ethernet phy,which failed to pass ethernet template. I combined small analog ground plane to digtal plane. Then I got good waveform.

I am not sure this is the only factor to improve a signal quality.

So I belive, in high speed design, solid big gound plane for both analog and digtal circuit is better than two small planes for them.

For ADC or DAC, the cirtial problem is not signal intergrity, but digtal noise. So I'd better seperate grounding.
 

ifarmer said:
When I design on PLL or PHY of Ethernet, I always treated AGND same as DGND, and lay a small AVCC in signal plane.

Is this method just easy to practise, or is also good for analog part?

In fact, I changed an interface board of ethernet phy,which failed to pass ethernet template. I combined small analog ground plane to digtal plane. Then I got good waveform.

I am not sure this is the only factor to improve a signal quality.

So I belive, in high speed design, solid big gound plane for both analog and digtal circuit is better than two small planes for them.

For ADC or DAC, the cirtial problem is not signal intergrity, but digtal noise. So I'd better seperate grounding.


If digital signal wire go through a whole plan,I think it doesn't matter.For ADC and DAC often have four side, two opposite side is for analog and digital signal.The others is for config. we can seperate ground cross this side.
 

in case if i have a bga in my design how i will fix the layers. minimum layer cam be calculated on what basics any idea ???

binu george
 

Minimum layers cam is normally calculated based on signal routing needs plus dedicated ground and power layers. You can split the ground plane layer into analog and digital ground sections and connect them at the ADC or DAC. Take precaution so the analog ground is not routed under any digital signals or components.
 

Try to cater for multiple ground connection points, using zero ohm resistors. When you test your board, try various connections to see if you see improvements in your circuit.
 

when i design motherboard , always need seperate ground plane.
 

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