[sdiy] filter cap needed?

[David G. Dixon]

Thanks to all for your replies.  I understood the basic idea about switching
transients.  I guess I'm just not convinced that putting the caps adjacent
to IC power pins is significantly better than putting them an inch or so
away on the power rail, near the ends of ICs.  I'd appreciate any comments
in this regard.  If it looks like this would make a significant difference,
then I might have to change my ways...

Cheers,
Dave

> David G. Dixon wrote:
> >>> However, something has been troubling me about these caps: If several
> of
> >>> them are connected between a rail and ground at various locations,
> >>>
> >> wouldn't
> >>
> >>> they end up fulfilling the same function as one cap, several times as
> >>>
> >> large
> >>
> >>> (given that they are capacitances in parallel)?
> >>>
> >>>
> >> If they are at different locations, there must be a conductor between
> >> them which has resistance and inductance.
> >> So, for high frequencies, no they wouldn't .
> >>
> >
> > What do you mean by "high"?  Assuming the trace represents a resistor R
> = 1
> > ohm (which is probably conservative), and the cap is C = 0.1uF, then
> this
> > makes a low-pass filter with a cutoff frequency of about 1.6 MHz.  How
> does
> > this help?
> >
> 
> What is meant by "high": anything with a fast switching transient - e.g.
> an LED that turns on and off at a rate of 1Hz will couple clicks into
> the power rails every 0.5s.
> 
> Think of it this way: PCB traces have some small resistance, and more
> importantly for high frequencies, some small inductance.  If you have no
> bypass caps then any high frequency current load (e.g. any kind of
> transistor or IC switching on and off, etc.) will create local changes
> in voltage on the power rails because of the resistance/inductance of
> the trace.  These changes in voltage can then couple into other parts of
> the circuit running off the same power rails.  By putting bypass caps
> right by the power pins of the IC the instantaneous current is drawn
> from the caps instead.  That's why it's also important to use the right
> type of cap - an electrolytic which has a high ESR would not do the job,
> you need something with low ESR like multilayer ceramic.
> 
> Aside from avoiding noise coupling between parts of the circuit,
> de-coupling caps also ensure things can switch on and off in a timely
> matter - this is of course essential for high frequency digital stuff,
> but also for stable operation of many analog ICs.
> 
> Admittedly if you're lucky you can get away without bypass caps in a lot
> of analog synth circuits, but given the cheapness and ease of sprinkling
> a few caps around, compared to the (sometimes subtle) array of problems
> it can cause to leave them out....
> 
> Seb