filtering high-frequency noise from power bus

[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?

 

[PeteS]

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?

In general, and without knowing details, a 0.01 uF to 0.1uF cap for each
two chips is a good idea. If you have extra induced noise, then one
small cap per chip close to the power pin should help.

Cheers

PeteS

 

[Chris]

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?

--

________ Jim Alexander __________________ [email protected] ________________
I have yet to see a problem, however complicated, which, when you looked at it
in the right way, did not become still more complicated. -- Poul Anderson

Hi, Jim. If you got a 50V spike on the power supply of 4000-series
CMOS, they'd be toast in a nanosecond. A voltage spike of lesser
magnitude on the power bus could easily trigger the one-shots, though.
Look at your measurements carefully, and make very sure about what
you're actually seeing with your scope. Always believe what your
instruments tell you until you've proved otherwise -- especially if it
doesn't make sense.

Tracking down noise problems to make things work can be an excruciating
experience, for sure. And getting the power supply right is definitely
a first step.

I'm assuming you've got a power supply external to the board, at some
distance away. Before anything else, hook your power supply up to
another load, and see if it still misbehaves. What you're describing
sounds a little bit like a glitchy switching power supply. It may need
to be replaced.

Assuming the power supply is OK, put a good elecrolytic cap (100uF to
470uF, made for low loss at higher frequencies) at the point where the
power supply connects to the board. Then you want to use a good 1uF
tantalum for every row of several ICs, and an 0.01uF to 0.1uF ceramic
cap across the power pins of each IC. 4538 monostable multivibrators
are very susceptible to glitches in the power supply, so you might just
want to protect every IC that way.

Once you've done that, you should use your scope (put it on 10X) to
look at the power again, and see if it's better. If you've still got
problems, try replacing the ICs.

By the way, make sure *all* unused inputs are tied down -- a floating
reset input could do this, too.

Long term, though, I'd recommend you look at the inputs to the ICs --
they're not protected against ESD or inputs beyond the supply rails,
and that may be messing things up. If you're using these inputs to
recieve signals from off the board, this could be a big problem.

Please feel free to post again with more information.

Good luck
Chris