Re: gEDA-user: General ground/power plane design questions

[Randall Nortman <geda-lists@wonderclown.org>]

On Thu, Oct 21, 2004 at 06:40:52AM +0200, J.D. Bakker wrote:
[...]
> - http://www.ultracad.com/ has a few articles/tech notes on PCB 
> layering, EMC et al. Well worth the read.

I will check it out.  Thanks!


> - You can easily do this design in a 2-layer, even if you need 20+ 
> bits resolution. For comparison: more than a few semi-professional 
> audio ADC designs get 20-22 bits out of a 2-layer design, usually 
> limited by source/component noise and not interference/crosstalk.

Yeah, my prototype batch of boards was 2-layer.  I had a lot of
routing problems, and lots of vias.  The auto-router in pcb was, alas,
almost useless, except in limited circumstances.  I'm seeing some
occasional weird behavior on some circuits, which may or may not be
attributable to anything that would be fixed by power planes.  The
price difference to bump up to 4-layer is $20/board, which in my
circumstance is acceptable, because even if it only provides slightly
better performance, it will be a lot easier.

> 
> - If you need really noise-free relays, consider using latching 
> (electromechanical) relays. You only need to supply dirty digital 
> current when switching; 'real' relays usually have much better 
> coupling/distortion properties than semiconductor ones. Especially 
> useful for settings that hardly ever change, like input attenuation 
> or input selection

What's the MTBF of electromechanical relays vs. solid-state?  Most of
these relays will be switching ~4 times per hour, 24x7x365.

[...]
> - Similarly, use the slowest logic family that works for your design. 
> If a 74HC chip meets your timing requirements, don't use a 74LV chip.

Can anybody point out an up-to-date description of all the logic
families?  My old copy of The Art of Electronics (Horowitz & Hill, 2nd
edition) doesn't have everything I see in the catalogs these days.

> 
> - Use differential input signals. 'Nuff said.

That would be enough said if I were actually a competant engineer, I'm
sure.  ;) Are you talking about the analog inputs, or digital signals?
My long-distance digital communications are differential (RS485), with
shorter runs of RS232.  Long-distance analog signals use 4-20mA
current loops.  But on the board, I'm limited to what my chips are
intended for -- i.e., CMOS-level pulses.


Thanks!

Randall