Bypass cap's

[Arthur Harrison]

SV:

>How many of you use bypass capacitors on your circuit designs?

AH:

Used when needed, but their application must be evaluated for each case.

SV:

>Where do you put these in the circuit and what size caps do you use?
>(5-10 microfarad?)

AH:

Again, this depends on the circuit.  A typical 10uF aluminum electrolytic
may do
little to reduce supply transients or ringing at certain frequencies, due to
its inductance.
Often, a parallel combination of different capacitors are needed.  High
frequency
designs may require the use of ceramic chip caps connected as close as
possible
to both the IC's supply pin and a ground plane.  It is wise to consider the
capacitor's
self-resonant frequency to ensure that it acts like a capacitor at the
frequencies of
interest.  Generally, it doesn't hurt to incorporate a 0.01uF to 0.1uF
ceramic capacitor(s)
in each device socket, and a 10uF tantalum capacitor(s) at the board's power
entry.

SV:

>Is noise to and from the board via suply lines really that much of a
>problem?

AH:

Noise may be conducted or radiated, and must be evaluated not just in terms
of
the power supply currents in supply lines, but also within a circuit itself.
Decoupling
capacitors are never a cure-all for noise problems.
You must also consider proper load distribution, ground currents, and
shielding to
prevent noise problems.  Beside noise, bypass capacitors also address
requirements
for stability (i.e., preventing oscillations).

-Art