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Vdd and gnd for Mixed signal layout

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leohart

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Hi all,
1)Does normarlly mixed signal layout use two independent supplies?i.e.,vdda,gnda&vddd gndd,four pads on die and connect to four pins when packaging,then we connect vdda vddd and gndd gnda on pcb level.

2)If we can only use one pad for vdd and one pad for gnd on die,is it enough only to connect digital/analog gnd at one place and make its distance to gnd pad as short as possiable?I think this only prevents crosstalk at gnd,we should do the same thing for digital/analog vdd too,or otherwise,crosstalk will happen at vdd!

Any comment will be helpful!THX
 

A.Anand Srinivasan

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the two ground and power plates are separated to avoid noise pickup between the two especially the switching noise of digital circuits could be harmful to the operation of the analog part...
 

    leohart

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srieda

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leohart said:
Hi all,
1)Does normarlly mixed signal layout use two independent supplies?i.e.,vdda,gnda&vddd gndd,four pads on die and connect to four pins when packaging,then we connect vdda vddd and gndd gnda on pcb level.

2)If we can only use one pad for vdd and one pad for gnd on die,is it enough only to connect digital/analog gnd at one place and make its distance to gnd pad as short as possiable?I think this only prevents crosstalk at gnd,we should do the same thing for digital/analog vdd too,or otherwise,crosstalk will happen at vdd!

Any comment will be helpful!THX
Number of pins is driven by a lot of issues... Business implications are very high..
But at times, you ought to keep separate pins keeping the noise implications in mind.. For example if we have an in-built PLL, then its always advisable and sometimes mandatory to keep separate pins for pll supply..
so thats a very tough question... :|
 

    leohart

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leohart

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ok guys,I knew had better to use seperated pins for both two gnd and vdd.

But What I really want to ask is supposes we have to use one gnd and vdd,then we should connect both digital/analog vdd and gnd only once and as near as possiable to the pad.it is not enough If only the gnd is treated so cause crosstalk will happen at vdd.

Am I right?
 

srieda

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Yes you are right..
Connect them as close to the bond-pad as possible..
 

A.Anand Srinivasan

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ya you are right... also the two Vdd and ground planes are connected through capacitor if possible to avoid bounce....
 

betty1007

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the two ground and power plates should be separated to avoid noise crosstk
 

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