Re: Separate +5V Plane

Andy Ingraham (ingraham@wrksys.ENET.dec.com)
Fri, 28 Mar 97 13:55:12 EST

Farrokh Mottahedin writes:

> It is a common practice to separate the GROUND planes between digital
> and analog portions of a design. I have heard pro and con arguments
> on this practice. I am interested in your opinion on this and also on
> the following questions:

There are times when separate grounds are appropriate, and times when
they are not. Analog IC manufacturers that do A/D, D/A, and S/H chips
sometimes talk about this sort of thing.

You also must be careful about WHERE to draw the split between grounds.
Remember, "no" switching signals should cross over the split.

> What about separating the Vcc plane between an area of a design with a
> noisy ASIC and another area where noise on Vcc can not be tolerated?
> Is it better to isolate the Vcc or to leave it a continous plane? Are
> there any application notes that might help me?

I think that separate grounds and separate VCC go hand in hand ...
where you do one, you must do the other.

For one thing, if you cut only GROUND, and GROUND and VCC run
parallel, then the intrinsic capacitance between VCC and your two
grounds will couple all the noise from one to the other that you
wanted isolated.

Derek Walton asks:

> PS Anyone know of a good way to share graphics over the mailing list? Would
> allow me to distribute some emission curves.

Most graphics files tend to be large, and binary. Binary files can be
"uuencoded" so they will go through generic mail systems. But please
do not post large files to the mailing list!

Binary file attachments also aren't a good idea, for the same reason,
plus a number of us work behind "firewalls" or with older mail systems
that don't do attached files. Even if the mail reflector can handle them.

If you have a personal web site or FTP accessible directory, put the
graphics files there and post a pointer to it.

If not, let people know you can e-mail the files to people who request
them.

Regards,
Andy