From: Sar Saloth (sarsaloth_at_yahoo.com)
Date: 2002-03-14 14:52:30
At 06:30 AM 2002-03-14 +0000, you wrote:
>--- In buildcheapeeg_at_yahoogroups.com, Sar Saloth <sarsaloth_at_yahoo.com> wrote:
>
> > I will comment (ha, you knew that would happen). You can increase
> > the common-mode impedance by replacing your second stage pi filter
> > with one that includes in inductor in the ground (just like your
> > input stage with the beads). Even for commercial medically
> > approved supplies, common mode noise is a big issue, and even when
> > they claim that the supplies meat everything, sometimes you have
> > to add more common-mode impedance.
> >
> > If you want to see proof - sorry, I am not able to provide any.
>
>Gee, I suppose I will have to put in that extra $2.50 toroid on pure
>faith then. ;o) I was afraid I was overdoing it, not the other way
>around...
>
>Since we're on that note: how about putting each filter stage in a
>shielded enclosure of its own, at least for the first three stages? A
>sort of staggered shielding if you will.
Point taken, no need to get snarky, even if I do deserve it.
I wasn't actually thinking of $2.50 toroids, I mean isn't that close to the
price of a big ferrite sitting on a power cable anyway? (which is the last
minute fix that my well-intentioned comment was trying to eliminate). But
if you are really winding your own inductors on $2.50 toroids, why not just
add some wire and make a real common-mode choke for one of the earlier
input stages? EFT is one of the harder tests to pass anyway.
In fact, why use a toroid? (that is a real question, I have some
understanding of magnetics, but when it comes to fancy filtering, I just
use a sort of shot-gun method).
Really, I was thinking of some $0.10 ferrite beads instead of $2.50
toroids. I have never done a step-by-step test to prove which components
are useful.
>From a practical point of view, even if everything I said was correct,
(which we know it wasn't) my points are somewhat irrelevant. I mean before
a fortune is spent getting a product medically approved I am sure that it
would go through a few more revisions anyway, so why worry about some
details that won't really impact the observed behaviour of the device in
your application? I am trying to be helpful, and I realize that I may not
be because the time spent discussing the trivial points is time taken away
from getting a working item. For that reason, I will not be the least bit
insulted if you completely ignore suggestions. I am just trying to help
because I am impressed with the work being done here and I will see if I
can use any of it towards my own purposes.
To save you time, if I see something that really does look like a mistake,
I will use strong language in my suggestions.
And in the off-chance that your suggestion was not sarcastic, I can tell
you the following from experience. There are some medical devices on the
market that have no shielding whatsoever. (The Stardust portable sleep
recorder is an example, although it has no EEG inputs, it does have a pulse
oximeter which is a very noise sensitive device). Many other devices have
shielding in the box (or a shielded box) in order to pass the 3V/m radiated
susceptibility.
If you wanted to be ultra paranoid, you could always lay out your board to
take your shielded ideas as an option.
My suspicion is that many devices with no shielding at all are improperly
tested and approved. I have seen one EMI test report from a European
notified body for a medical device that clearly did not understand the test
specifications. Some other Europeans I had spoken with also didn't
comprehend some of the important points of the test specification (most
importantly the requirement to modulate the carrier to at least 80% with a
frequency approximately in the middle of the pass-band for the device under
test). How important this is to your current work I leave to you. I can
say that one company I was involved with spent many times my mortgage on
getting approvals for one device that had extremely low criticality.
Just trying to be helpful - Sar
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:40 BST