J
Jim Alexander
I have a device involving several 4538 CMOS monostable multivibrators that
has been operating fine for 6 months or so, but these multivibrators have
suddenly started triggering spontaneously. I hooked up a scope, and found
nothing unusal on the leading-edge trigger line, the one I am using, but found
that these spontaneous triggers are correlated with some high-frequency noise
on the 5V VCC bus. The unused trailing-edge trigger input is tied to VCC,
which is what the spec sheet calls for. The noise has a period of about 20ns
(50MHz) and a typical duration of a few hundred nanoseconds, and the
voltage ranges from 10 to 50V peak-to-peak. I think this noise is causing
these unintentional triggers.
I confirmed that this noise isn't coming in via the regulated 5V power
supply, so I'm guessing that it's induced via EMF from an unknown source.
I'm looking for advice for getting rid of this noise (as is probably obvious,
I'm inexperienced with handling RF interference). Is there some way I can damp
out the noise on the whole power bus, or am I better off filtering at
the affected inputs? Or is shielding a better approach?
has been operating fine for 6 months or so, but these multivibrators have
suddenly started triggering spontaneously. I hooked up a scope, and found
nothing unusal on the leading-edge trigger line, the one I am using, but found
that these spontaneous triggers are correlated with some high-frequency noise
on the 5V VCC bus. The unused trailing-edge trigger input is tied to VCC,
which is what the spec sheet calls for. The noise has a period of about 20ns
(50MHz) and a typical duration of a few hundred nanoseconds, and the
voltage ranges from 10 to 50V peak-to-peak. I think this noise is causing
these unintentional triggers.
I confirmed that this noise isn't coming in via the regulated 5V power
supply, so I'm guessing that it's induced via EMF from an unknown source.
I'm looking for advice for getting rid of this noise (as is probably obvious,
I'm inexperienced with handling RF interference). Is there some way I can damp
out the noise on the whole power bus, or am I better off filtering at
the affected inputs? Or is shielding a better approach?