At 08:12 AM 8/10/98 -0400, Larry Campbell wrote:

>In all probability, the older boards you are going to convert, are
>analog circuits and don't require the same expertise that the
>boards of today require, with their higher speeds and digital
>circuitry.

It's a little dangerous to tell this to a beginning designer. Right, analog
boards don't require the "same" expertise. They require a *different*
expertise. *Some* low-frequency analog boards are very simple. Others
require great attention to loop area, for example, or power routing.

Digital boards are often more "difficult" than analog boards because of
sheer complexity, not because of the level of design knowledge required.

With the analog board and its discrete components, it can be easy to place
components in a logical position, thus keeping signal paths to a minimum.
But with many digital boards, particularly low-speed digital, one can often
simply place the components in more-or-less optimal locations (as shown on
a rat's nest) and then let an autorouter finish the board. This style of
design can be a disaster with analog boards.

As far as a rat's nest is concerned, a feedback resistor might be placed
close to the output or the input. But from an analog noise point of view,
it should be near the input; and, in general, long low-impedance traces
should be preferred to long high-impedance traces. With analog design, one
*must* pay attention to the schematic and the function of the components.

Mr. Campbell gave many other items of excellent advice....

[log in to unmask]
Abdulrahman Lomax
P.O. Box 423
Sonoma, CA 95476

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