Re: gEDA-user: Question for DJ about his alarm clock

[Ben Jackson <ben@xxxxxxx>]

On Fri, Aug 15, 2008 at 04:58:32PM -0400, Stuart Brorson wrote:
> >> I noticed that on p. 3, the ST part (U300) has PVDD and PVSS
> >> connections through 4.7 ohm resistors to PWR and GND, respectively.
> 
> Did you use a ground plane?  If so, then I'd expect that the plane is
> quiet, and it would be better to connect directly.  Or am I wrong?

If the device consumes variable power then grounding through a resistor
is going to *add* noise.  Even in datasheets recommending the most
paranoid decoupling schemes (like a bead + 2 or 3 decades of caps per
power) I've never seen one suggest anything other than a direct ground
connection.

And even if there's AGND and DGND it's critical that they be at the
same potential or current can flow through the substrate of the chip.

I highly recommend:

http://www.amazon.com/Linear-Circuit-Handbook-Engineering-Devices/dp/0750687037

It's a collection of wisdom by Analog Devices engineers.  It's one of
the most practical books on actual circuit design I've ever seen.  Where
real life differs from ideal components there are clear guidelines based
on experience instead of deep dives into differential equations.  It
also explains enough of the internals of modern ICs (such as DACs) to make
it clear where seemingly arbitrary datasheet recommendations come from.

-- 
Ben Jackson AD7GD
<ben@xxxxxxx>
http://www.ben.com/


_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user