Thanks for your reply, Okay, I guess that "leads" me to another question or two. How should I tie in the 2nd ground pin on an F876? And why is it even there for that matter? I was just going to connect it to the other ground pin by running a trace underneath the part between them. I had did a search in the piclist archive, and kept coming across the term "vias". What is a "vias"? I'm thinking it must be a euphemism for traces or something. I believe I read one post in there about using thinner ground traces for high frequency digital, so that it would cut down on the amount of frequency that gets back to the central connecting point. -----Original Message----- From: Dan Michaels To: PICLIST@MITVMA.MIT.EDU Date: Saturday, March 31, 2001 7:55 PM Subject: Re: [EE]: Seperating grounds >BBob wrote: >>I've seen it mentioned here a few times about "seperating" the digital and >>analog ground plains from each other. >> >>Since everything eventually has to be connected to the same common ground >>somehow (at least in my circuit), how exactly is that done? I mean, how does >>one keep the digital signals from getting mixed into the analog ground plane? >>With a signal diode, a cap? An example diagram (draw me a picture ;-), or >a URL >>to a page/circuit that outlines that would help. >> >>Sorry for the stupid question, I've never had to worry about this in my little >>analog circuits. >> > >Bob, > >Not stupid - this is a basic problem when mixing analog and digital. >Less important for slow signals and low sampling resolution [ie, 8-bits]. >More important for faster signals, high-sensitivity [ie, less a few mV], >and high-resolution [ie, >= 12 bit sampling]. > >The general solution to successfully mixing analog and digital probably >doesn't exist, but everyone has a dozen or so partial solutions, and >there are "very long" threads about this on piclist - if you can find >them. This whole area is probably more art than science. > >"High-Speed Digital Design" by Howard Johnson is one place to look. > >The basic idea to separating digital and analog grounds is simply >just that - keep them physically and electrically separate from >one another all over your circuit, and tie them together at ONE point >only - at the place where the power comes onto your board. This point >usually has wide traces with low impedances [meaning the digital currents >will devleop relatively low noise signals --> Vnoise = Idigital * Z]. >Also, a "one-point" connection helps prevent ground loops. Think of >the various grounds emanating from that point kind of like the legs >of an octopus. That is the common solution. > >OTOH, if you look at the datasheets for various high-performance >A/D converters [downloadable from National Semi, Linear Tech and Maxim], >you will see many of them recommend making the "one-point connection" >directly under the A/D itself, which is usually not at the power >insertion point of the board. So this kind of goes against the common >solution just described. > >This being said, however, does not mean that simply separating the >gnds is going to magically make your analog circuitry noise-free. >This is not a stupid or trivial problem by any means - and many engineers >spend their entire lives trying to figure it out -[and some are still >working in it - he, he]. > >hope this helps, >- dan michaels >www.oricomtech.com >====================== > >-- >http://www.piclist.com#nomail Going offline? Don't AutoReply us! >email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > > -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu