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Analog & Digital Supplies

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kommanche

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Hello,

I am designing mixed signal board in which I use FPGA. I connect all the grounds (analog and digital) to a common ground plane and I want to feed 1.8 V analog and digital Vdd's from the same output a DC/DC converter. Although isolation is strictly adviced in datasheet, I think that feeding both from a common supply will not cause any problems because I already tied grounds together. May I have your comments please?

Thank you very much.
 

Re: Analog & Digital Supplies

It's a good idea to use a surface mount ferrite to provide some isolation, and each should have their own decoupling caps close to the pins.

- - - Updated - - -

Also, this article has some good advice for grounding in mixed signal designs.
 

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The degree of power and ground isolation you need depends upon the lowest level of analog signal you need to resolve. Connecting the analog and digital directly together will likely result in a couple hundred millivolts or so of noise in the analog signals. If you can tolerate that than use the layout you have.

If not then I suggest you follow the guidelines of the reference pstuckey posted, go with a separate analog ground plane (connected to the digital plane at one point) and ferrite and capacitor isolation of the power as pstuckey suggested. Digital don't play well with analog and you normally want to isolate them from each other as much as possible.
 
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