Kelvin@!!! said:
Hi:
how dose bypass capacitor filter out the noise on the signal?
say i have a 74HC74 chip... with a bypass capacitor connecting the Vcc and
Gnd, i get a really nice wave form. but w/o one, i got noice all over the
place! can't even get a waveform on the scope...
it's like magic!
i know how a cap can filter out high freq. signal. but just can't figure out
how a bypass cap work....
thank you for any answers...
Wires have inductance. Circuit board traces have inductance. It
takes time for current to make its way from a power supply to a chip
that has a sudden drop in resistance across its supply lines (and
modern chips can do this extremely quickly, compared to the time it
takes light to make its way to the supply and back). Inductance takes
time for current to change by the differential equation:
V=L*(di/dt). A capacitor is defined by the differential equation:
I=C*(dv/dt). This means that the current through a capacitor is
proportional to the time rate of change of the voltage across it.
(Whew!)
So taking all that into account at the same time (imagination is
faster than the speed of light) when a chip passes a sudden pulse of
current, instead of that pulse having to make its way through the
circuit board trace inductance and supply wiring inductance (including
the time it takes for the current to change through those inductance
and not even worrying about the speed of light over that distance) and
having to put up with all the voltage sag it takes to drive that
current pulse through all that inductance, if you put a capacitor very
close to the chip, and if the capacitor is big enough, a small sag in
the capacitor voltage allows it to supply a very quick pulse of
current to the chip.
Then the inductive supply distribution system charges the cap back up
before the next pulse.
Isn't that easy?
;-)