From oldcrow@access.mountain.net Wed Nov 22 11:35:29 1995 22 Nov 95 14:35:12 +0500 10 Oct 95 19:31:56 +0500 Date: Tue, 10 Oct 1995 18:45:40 -0400 (EDT) From: The Old Crow Cc: "'bode@veda.is'" Subject: Re: Wiring questions On Mon, 9 Oct 1995, Dan Higdon wrote: > I've breadboarded an LFO and VCA; they seem to work fine except that > the signal "pops" on every LFO cycle. I figure this must have something > to do with either my power distribution or coupling (I'm guessing the former). > > So, what's the best way to feed power into a module, given that all modules > are being driven from a central power tap? Should I put a large cap between > each power line and ground on each module? If so, how big? Usually it is best to provide at least 100uF to common for each supply voltage at the entry point on the board. If the circuit is sufficiently large, additional 100uF caps located at far points of the circuit (say, the opposite corner or side of the board w.r.t. the power terminals). Since I have bunches of them, I tend to use 220uF caps. Of course, make sure the WVDC rating is not exceeded; it is best to use the 'next higher' rated part vs. the supply voltage. (For example, I don't like to put a 220uF/16v cap on a 15v supply, or even a 12v supply. I would use 220uF/25v parts.) Many times I've found it beneficial to place a 68 to 100 ohm resistor in series with the power terminals of a board to the rest of a circuit. In addition, a free-wheeling 'snubbing' diode from the power terminal to common will shunt most impulse transients. Like this: from PS V+ >----/\/\/----+----+-----> V+ to circuit 100 | | from PS com >---+ _ | + (reverse diode & cap | 1N4001 ^ === 100uF for V- wrt to com) _|_ _|_ _|_ \ / \ / \ / It may also help to use 0.047uF or 0.1uF caps as decoupling to common as close to each IC as possible. I even sometimes put 10uF caps alongside 0.1uF caps on particularly sensitive chips. Finally, I ground plane most of my circuit boards (when I'm trying to achieve 0.1 degree Fahrenheit accuracy from an 18-bit converter there is no substitute), though depending on your resources this may not be doable. Regards, Scott /* Christopher S. Rider */ /* http://www.mcs.com/~syzygy/ */