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 <[EMAIL PROTECTED]> http://www.ben.com/ _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
