[sdiy] PCB layout question

Paul Schreiber synth1 at airmail.net
Sun Nov 2 07:53:19 CET 2008


> Afraid you have the wrong Paul here!
> It's Paul S who has the the degree and
> - more importantly - 40 years or so of experience.
>

Well, more like 35 years but so does Mr. Fritz :)

a) if you are going to use bypass caps, at least use decent ones. Avoid the 
1960's 'ceramic disc' types, they have high ESR (effective series 
resistance), they fail as a dead short (ACK!) and I think they are ugly :) I 
use Kemet C410C104M5U, in the past also the AVX 'SpinGuard' which I think 
are like SA105E104M if memory serves. These 2 are specifically designed to 
be placed on 0.300 spaced pads, same as a DIP. Extremely reliable, I have 
used over 60,000 on MOTM alone, probably 50 million in my lifetime of 
designs and I can't recall a single failure.

Disc caps (speaking of 'low voltage ones', I have had OK luck with 1KV disc 
caps but they are made differently) almost put Sequential Circuits out of 
business, they referred to them as 'the green caps of death'.

b) bypass caps are *mainly* for fast switching digital chips. But in VCOs, 
there will be a fast-switching comparator, and I would certainly bypass 
that. As a rule, I bypass the +-15V on every other IC, sometimes every third 
IC if board space is an issue. I always bypass wide-bandwidth, fast op amps 
like the OP275GPs, and things that suck current like the LM311 and the NE555 
timer.

I would be the first to state that to bypass EVERY IC is silly unless it's a 
100% digital circuit going 50MHz or something. I think that it's more 
important to have 'bulk caps' (like 10uf) at the power input connector. For 
fast BGA parts, it is not uncommon to have *50* bypass caps on it (yes, 50 
and usually 0402 which are insanely tiny)

A 'bypass every other analog IC' rule is fine and easy enough. Mouser has 
them at $15 for 300 on tape (just ignore their generic cap photo, which is 
silly). Probably a life-time supply for most folks.

Paul S.




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