[sdiy] filter cap needed?

[Seb Francis]

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