Isolation between analog and digital power lines

[ee484]

Hi, I have heard/learned that it is good to separate AVDD/AVSS (analog) from DVDD/DVSS (digital).

I have wondering how this can be possible. (Assuming Nwell process). Since I am sharing a substrate on both analog and digital circuitries. How can I separate them so that analog circuits can get less noise from digital circuits?


Thanks,

 

[leohart]

I was told to do two or more seperated group vdd/gnd then use them from the most sensitive analog part to the the dirtiest digital part...connect them only at one place two prevent dirty signal from sneaking into clean part

 

[acbalbason]

hi!

use guard rings!

- al

 

[srieda]

Use guard rings...
Usually chips have separate pins for digital and analog grounds which are shorted outside the chip - on board. Thus, shorting them is inevitable...

 

[ee484]

Yes, shorting ground is inevitable in the end. However, we should separate analog vss/vdd from digital vss/vdd? Can even possible to separate vss for analog and digital? Guard ring meaning, ntap outside and ptap inside? Even in this case, n-mos inside a guard ring requires psub contact, thus eventually ground are all connected. But, WHY DO WE WANT TO SEPARATE THEM? DO WE WANT TO HAVE DIFFERENT PINS OUT FOR ANALOG AND DIGITAL GROUND?

 

[em1cr0nix]

if u only have one pin for vdd and a pin for vss, try to connect digital and analog power lines at the pad... this will reduce the possibility of digital blocks noise to mix with the analog signal because of bypass capacitors that are usually laid-out in the top-most hierarchy. isolate n wells of analog from digital and try to put guard rings to transistors that are directly laid-out to the psub.

 

[DenisMark]

Look at chapter 18.3 of Razavi's book.
For triple well process u can place sensitive to supply noise circuits in one NWELL and nmos in their PWELL(PWELL in NWELL) with be isolated from PSUB. Thus it's easy to separate supply rails of analog, digital and power domains.
If u have only double well process, all nmoses are located on same die. In this case u're needed connect bulk contacts for each nmos to less noise radiating ground, analog or digital depended on ur circuit. Ever it's better use separate "clean" ground with separated pin.
If u are suppose that ur circuit will generate a lot of supply noise, u should choose more sutuable triple well process (aka mixed-signal).

 

[Fady Atef]

Hello

you can use N+ and P+ guard ring and connect them to a clean node (pad)

 

[jecyhale]

Separate all the VDD, VSS and PSUB of Analog form digital.

Use the Nwell, P+ and DNwell as the guardring, the noise will be reduced.

 

[loosin]

Use P+ guardring and Nwell N+ guandring to surround the noise source(such digital circuit).If it is possible,connect the guardring to a clean vdd and vss. In most case,we seperate analog and digital vdd and vss in chip and connect them in the pcb.The reason is that the bond line act as an induct. It can reduce most influence of the noise from digital to analog via the outside connection.

 

[selvaraja]

use N+ring guard ring for digital circuits and P+ring for analog circuits.

 

[Paramjyothi]

The Analog and Digital Circuits are separated by Guard Rings and share their own separate native substrates(both the circuits do not exist in the same Nwell). They are placed far apart so that the noise from analog is not induced into the digital circuitry. We have separate bus lines and pins for AVDD/AVSS & VDD/VSS. Sometimes they common ground (VSS) too.

 

[andy2000a]

analog & digital in the same substrate . sometime noise couple from SUB

even use guard ring ..

use "away from "

 

[lsimeon]

i think it is somewhat similar when you are designing a PCB, it is true that you have to separate the digital to your analog supply, the reason is to isolate also the analog supply from the noise of the digital supply.. for you to do that you have to dedicate a layer for your analog supply and also for your digital supply separately(im referring here the main terminal/wiring).. It is not necessary to separate your analog circuit to your digital circuit they can share the same substrate and same layer, what you have to take care are the supplies and grounds...
The closer your ground to your signal layer the better...