updated 2003-08-08
(what's a "Gen" ?)
This page has general electronic construction tips (applicable to ciruits build with solderless breadboards, wire-wrap boards, and printed wiring boards). Do we need another page for tips specifically for PWB design ? [FIXME: seperate "design-time" tips from "assembly-time" tips]
See also:
Q: "What does "dd" stand for in "Vdd" ?" A: "dd" in "Vdd"
[The worst thing is ] putting a cap to ground (some arbitrary point in the ground system) from a signal line, to "smooth it out"... Grrr. Now the driver needs much more current on each transition, and the problem is worse.Resistance in series solves this problem, but it's not so convenient to implement usually. (Tip: high speed clock lines should always have a resistor at the source. Even if later you choose the value to be zero, though likely 100 ohms will be better)
(see "Spice provides signal-integrity clues" article by Ken Boorom, EDN, 2/18/1999 http://www.reed-electronics.com/ednmag/article/CA56674?pubdate=2%2F18%2F1999 for more details. )
Cirrus (2006) recommends:
"... In addition to standard mixed-signal design techniques, system performance can be maximized by following several guidelines during design. ...- Place a buffer on the serial data output very near the A/D converter. Minimizing the stray capacitance of the printed circuit board trace and the loading presented by other devices on the serial data line will minimize the transient current.
- Place a resistor, near the converter, beween the A/D serial data output and the buffer. This resistor will reduce the instantaneous switching currents into the capacitive loads on the nets, resulting in a slower edge rate. The value of the resistor should be as high as possible without causing timing problems elsewhere in the system."
-- from the 2006 datasheet for the CS5368 24-bit A/D converter
A few more tips at Massmind: Avoiding Noise.
Although IPC-D-275 standard has been superseded by "IPC 2221A - Generic Standard on Printed Board Design" and "IPC 2222 - Sectional Standard on Rigid Organic Printed Boards" , as far as I can tell the new standards have the same recommendations as the old standards.
Also, don't use it because some silicone sealants contain acetic acid (smell 'em if you don't believe me) which will react with copper or iron, causing corrosion. Corroded copper and rust are famously poor conductors.
Another possibility is drilling two extra holes at the board (little bit bigger than the wire diameter), just to zig-zag the wire (snake path) through them after soldering. If you don't use any mechanical lock to the wire, sooner or later it *will* break up at the solder point.
Jinx wrote:
If you DO end up just soldering the three wires to the board, the back presumably, put a small dab of silicon sealer over the wire adjacent to [but not in contact with] the solder point. This will hold the wires if you're worried about movment loosening them but will still give access to the joint. Silicon holds fast but is very easily cut off with a scalpel.
Just a quick look at a Diodes Inc. catalog shows that the shottky's are typically 1mA reverse current, and the regular diodes are typically 5uA reverse current. I found out the hard way when I built a circuit that needed protection from negative going signals. I put the Shottky across the diode in an RC filter between two op amps, so the resistor served two purposes. I didn't look real hard when I tested it, and the customer complained about temperature dependency. We quickly discovered that when the cover was removed and a heat gun was used, an easily observed error at ambient became a nearly zero signal level out at high temperatures. The figures shown above for leakage are at 25C! Needless to say, it was a rude awakening to the drawbacks of Shottky.
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Greg Pulley says: rule one: don't worry about what it looks like.
rule two: make all bus bits the same length between parts
rule three: run clocks and critical control signals as point-to-point direct as possible. if they are above 5-8MHz, twist them with a dedicated ground wire (look up how to make twisted pair with a cordless drill).
rule four: run power and ground wires first. add plenty of 100nF decoupling caps directly across the parts. It's also good to use tall sockets and solder the decoupling caps underneath straight across the power pins. Add 10uf bulk electrolytics every 5 chips. add 100uf at the input to the circuit where the PSU comes in. ferrite beads on your power supply leads are good idea too to reduce EMI.
rule six: breadboards suck if you want whatever you build to work more than 2-3 days without becoming flakey. Use protoboards and solder, or if you have the tools, wire-wrap is a good choice too. For WW, dedicated power grids made with 14-16AWG wire are best.
Don't be all that concerned about the neat breadboards people post - the high-speed performance is terrible, made worse by all the mutual coupling and parasitics beyond what the crappy breadboard saddles you with.
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