The comparator I use will stand 36 volts.
All leads are either zener diode protected or have 346K resistors in series.
All this RF stuff is over my head but the circuits were designed by a
engineer who deals with it daily.
Ode
At 01:17 PM 6/16/2003 -0400, you wrote:
url: http://escribe.com/health/thesilverlist/m60257.html
Re: CS>$$$ perpectives
From: Mike Monett
Date: Mon, 16 Jun 2003 06:48:22
I wrote:
CMOS is very sensitive to esd. TTL draws more power, but is pretty
much immune.
The headaches really scramble my thinking. This statement is true, but
useless in this context.
CMOS can be damaged or destroed by ESD events. There is protection at the
inputs, but it is only good enough to pass the industry test so the
product can be shipped. It will not survive the strong esd events that
can occur in ordinary use.
Another problem is an esd event can cause the device to go into latchup
and be destroyed if the power supply can deliver enough current. Usually
several hundred mA will do it.
This is what I was thinking of when I said TTL is immune. TTL cannot go
into latchup, but it is most certainly affected by esd and rfi.
However, the comparator inputs are not TTL. I don't know which device you
guys are using - very few that can withstand 30 volts differential across
the inputs.
If the device is CMOS, I'd make sure there was very good protection at
the inputs, especially on systems that have long leads.
The same filter techique to protect against rfi works for esd. But you
really have to check it carefully. ESD risetimes can be sub-nanosecond
with currents in the hundreds or thousands of amps.
The problem this causes is the voltage can arc across the series resistor
at the input. If the bypass cap inductance is high, or the traces are too
long, this can allow very high voltage to appear at the input to the
device. This can damage the thin oxide or create latent defects that fail
months later.
You will never know why. It won't occur often enough to seem to be a
serious problem. But each time it happens, someone will lose their unit
until it is repaired.
However, rfi or esd protection can ultimately be defeated. No matter how
much you install, there is always some place where it won't work. Walking
across the carpet in Colorado during the winter can draw 1 inch arcs.
This is over 30kV. That really takes good filtering to minimize the
effects. You will never eliminate them completely.
If you are in an industrial area, being next door to an arc welding
outfit is not a good idea. If there is a high powered transmitter nearby,
it can also cause havoc with sensitive comparators.
I once found a huge problem when a laser company moved in next door. They
used 10 KW heaters to melt the glass. The temperature was controlled with
a simple bimetallic thermostat.
When the contacts opened, they produced a small arc. This resonated with
nearby wiring and pruduced a huge spike at 40 MHz. I meaured the
frequency by triggering a scope and seeing the rf pulse in my lab. This
severely disrupted the product I was working on.
The cure was simple in this case. A snubber across each contact provided
enough damping to kill the arc. The snubber was a 47 ohm resistor in
series with 0.1uF right at the contacts.
But it took a while to figure out where the noise was coming from. And
there will always be the next one...
Best Regards,
Mike Monett
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