Separate ground lines for analog and digital sections

[minhchau]

Hi,
My chip has the digital section and analog section, to reduce influence noise from digital, the power and ground are separated.
I can see a real effect when it comes to power separation, but separating ground doesn't seen to have much effect.
My set up DVSS and AVSS to verify separating ground showed as below:

DVSS is ground of digital section (2 bonding wire)
AVSS is ground of analog section (10 bonding wire)
And there is simulation result:

The amplitude of AVSS is not much difference between combing and separating ground. So that, the performance does not change.

Is this result reasonable?
Are there somethin wrong with test bench?
Thanks

 

[FvM]

It's not clear how your "combined ground lines setup" exactly looks like. Common ground bond wires? Different ground metal layout than the separated setup?

Even more interesting, which "noise" sources do you consider? According to wave forms, you have either similar noise sources in the analog section, or other on-chip coupling pathes between digital and analog section.

Does your setup involve IO currents? Strongest effect of ground wiring is caused by ground bounce, of course only occuring with clock sources and loads connected.

 

[minhchau]

It's not clear how your "combined ground lines setup" exactly looks like. Common ground bond wires? Different ground metal layout than the separated setup?

Even more interesting, which "noise" sources do you consider? According to wave forms, you have either similar noise sources in the analog section, or other on-chip coupling pathes between digital and analog section.

Does your setup involve IO currents? Strongest effect of ground wiring is caused by ground bounce, of course only occuring with clock sources and loads connected.

Thanks FvM,
When combining ground, the ground of digital and analog will conenect together in the chip, and this ground is bonded to PCB (12 bonding wire) and when separating ground,the ground of digital and analog will seperated.

I consider the noise from digital, there is no path between digital and analog section (except bit control bus). I also find it difficult to understand why when separating the ground, the noise in AVSS doesn't change.

Yes, but no clock source and loads connected