digital vdd and analog vdd?

[ee484]

Hi, I understand that it is better seperate between analog vdd/gnd and digital vdd/gnd.

Typically, all pads have ESD protection. Thus, PAD has pad ring that provides vdd and gnd.

Should I separate ESD vdd and gnd from other power/ground?

In the process I am using, it has digital power pad and analog power pad - all pads are associated with ESD diodes. In the layout, all ESD diodes share vdd and ground without distinguishing analog and digital.

I found that I have to cut this power/gnd line so that I can separate digital vdd/gnd and analog vdd/gnd.

In the library I am using, there is cells called "power cut" cells. Some names that associated with these cells are DIODE_DIODE, SNAP_DIODE, DIODE_SNAP, SNAP_SANP.

Do you guys know what are the differences between them?

What is SNAP? It is defined as "low voltage REVERSE". I don't understand what it means.

Is there any body who has experience using POWER_cut??

In my PADs case, VDD and gnd that I use must be also used for ESD vdd/gnd?
Any other way, you would like to recommend??


Thanks in advance!!!!

All replies will get "helped!" click!

 

[anilsaini]

seperate vdd

As far as my knowledge is concerned, In mix signal chip supply pin are divided in digital, and analog part, means separate Vdd & Gnd pin for both digital and analog circuitry. Because, digital circuit makes a noise due to fast switching and this noise directly effect analog circuit performance. But these separate both vdd (and Gnd) pins are connected with each other on PCB.

 

[vaibhav.k]

placing analog digital pads

For Digital Supply and Ground There is switching noise in forward i.e. supply and return line i.e Ground due high frequency level transition .If u will mix that with analog supply and ground in near the controller it will effect your ADC performance.If possible please provide different supply regulator for analog section.And merge that sections ground to System ground on terminal of Coupling capacitors through inductor.

 

[DenisMark]

separate vdd vdd

"Should I separate ESD vdd and gnd from other power/ground?"
Of couse, 1) esd rails are noisy because all i/o buffers connected to them 2) during esd zap sufficient ir drop may exist on esd rails, so this addtition can cause breakdown of core ckt.
"In the process I am using, it has digital power pad and analog power pad - all pads are associated with ESD diodes. In the layout, all ESD diodes share vdd and ground without distinguishing analog and digital."
ESD stress applied to this pads flow thoung diodes into ESD protection network. There must to be existed clamp circuit (better RC based) that pass ESD stress.
" "power cut" cells" only serves to separate core supply rails from esd rails and share their clamps for maximum esd protection. Almost the same action as diodes on dig./anal. pads.
"What is SNAP?" - snapback. Turn-on of parasisitcs npn (or pnp) bipolar transistor.
"In my PADs case, VDD and gnd that I use must be also used for ESD vdd/gnd? "
Connect esd rails only near dig. pads. Only on the pad!!! Use powercut cell to share esd protection from esd/dig rails to analog part. Use CLAMPs. Usually powercut includes antiparallel diode strings. Shorter string means better esd perfomance, but more noise penetrate from io and dig. part to analog part.

Cool book

 

[leohart]

how to isolate the analog and digital power

so we can conclude below:
1)two seperated vdd/gnd pads pairs for analog and digtial,and they are connected properly on pcb
2)esd's power/gnd rail uses digital supply system only, and connect them only on digital pads
3) ... (I don't understand the powecut part,seems it should be between esd and analog supply to isolate the noise and provide esd protection?)

PLZ correct me if I made mistakes and finish this list...

 

[arsenal]

digital and analogue vdd

on the top level layout, analog power rail should be abut each other and so is that for digital power rail. So for different voltage domain, that is, u dont wanna connect the analog power1 to analog power2 by abuting together, then u can use power cut cell to make it seperated.

arsenal

 

[DenisMark]

separate analog and digital supply

"(I don't understand the powecut part,seems it should be between esd and analog supply to isolate the noise and provide esd protection?)"
Let think a little and consider some possible ESD zap conditions.
Cond: ESD zap between an.vdd and an.gnd. Solution: place CLAMP between an.vdd and an.gnd and/or use powercut cell to use CLAMPs located on esd rails.
Cond: ESD zap between esd.gnd and an.gnd (or esd.vdd and an.vdd) Solution: place powercut cell between ones, thier pass ESD zap

Powercut doesn't isolate the noise, it pass them from periphery to analog. part. ESD perfomance and this noise are in trade-offs.

 

[parthi2311]

Re: separate vdd vdd

Hi Denis Mark,
Can you please provide the link once again cos the above link is not working. Thanks in advance.