From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-01-07 19:45:43
Dear Jim and Group:
----- Original Message -----
From: Jim Meissner <jpmeissner_at_mindspring.com>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Sunday, January 06, 2002 9:21 PM
Subject: Re: [buildcheapeeg]Patient Safety and Input protection
...
> I have been following the Input Protection discussion, and in my
> opinion it is going in the wrong direction with the relays, etc.
>
> The FIRST consideration should be for the patient safety rather
> than protecting the input amp.
That is exactly my opinion.
> The brain wave monitor must be safe so that if you are hooked up
> with the electrodes on the head and touch a live 220 volt AC wire,
> you should not electrocute yourself, rather you should not feel
> any sensation. This is what the 10 micro amp spec is attempting
> to guarantee. If the brain monitor is battery operated and has a
> good opto isolator, this is EASY to do. This is a MUST in my
> opinion. ( BTW most isolation transformers have too high a
> capacitive coupling)
What do you think of the TMA0505S DCDC converter ? It has
low coupling C and high isolation R)
> You may say that this can never happen.
In sufficiently users*hours it _will_ happen
someday...
...
> EEG electrodes. After a few very unpleasant shocks I added the
> opto isolator. ( I have hand drawn the schematic and listed it on
> my website if you are interested in "my" solution. = "Iso01.gif" )
>
Can it handle speeds of 56 kbaud ?
> The way that the isolation current can be measured is quite easy.
> Hook all the inputs and the chassis ground of the brain monitor
> together and connect a 1 meg resistor to the hot 220 volt line.
> Have a battery operated true RMS voltmeter measuring the drop
> across the resistor. That tells you the leakage current at 220
> volts.
>
> If you want to be "really" safe meeting this leakage spec at 5,000
> volts, you can use Brian's circuit using QSE158-ND ($1.42, qty
> 1)and QEE113-ND ($.74, qty 1) with 1 or 2 inch spacing between the
> LED and photodetector on separate PC boards. ( I really like
> this. )
Can it handle 56 kbaud too ?
> The SECOND consideration should be for the input amp. You have
> looked at "my" input amp design and probably noticed that I had a
> 10 K protection resistor in the input to the op amp.
> Unfortunately I had to short it out because it contributed too
> much noise.
Hard to understand, why 10K should contribute much noise,
because the TL084 has JFET input stages with very low current noise
...
> If the opto isolator is working properly, there really is no
> pathway for the current to go to. I found that the collector to
> base diode of the 2N3904 transistor provided adequate
> protection for the op amp input stage. As you guys are
> discovering, it is difficult to find a diode that will work in
> that location. The only job that that diode has to do is to
> divert the occasional
> electrostatic charge pulse away from the input stage.
> In the 10 years that I used this circuit, I have not had a
> failure.
>
> The THIRD requirement of dealing with a catastrophic failure of
> the op amp, is in my opinion a mute point.
Is it really ?
> If you properly clamp
> the transients and protect the op amp inputs, it is not likely to
> fail. If it were to fail for some reason, you might apply 5 volts
> or so to the electrodes going to the head. I would be difficult
> to perceive or have a sensation at that voltage. Certainly no
> damage would be done.
Lets assume a properly attached commercial EEG-electrode with 5k resistance.
Then we would get for a catastrophic failure and +2.5, -2V supply (e.g. modularEEG,
RS232EEG) I=5V/(2*5kOhms) = 0.5mA or 500uA.
This exceeds the IEC / FDA limit of 10uA by factor 50x.
This will probably not kill the user but could result
in electrolytic damage of the skin especially if the problem
is undiscovered for many hours (e.g. sleep monitoring)
>
> Good shielding and RF bypassing is also important. I believe using a
> (+) and (-) power supply allows ground to be ground for RF
> (radio stations), DC, electrostatic pulses.
> A floating ground requires some expert consideration of impedances at various
> frequencies.
The ground (exactly Vref/2) of the modularEEG is not "floating".
It is connected to the low-impedance output of an op-amp and
heavily bypassed to +5V and 0V.
> Too much trouble for my likes. A second battery is simpler and more robust.
> Also I found that the amplifier worked
> much better at +/- 12 volts than +/- 5 volts.
For common mode rejection high operating voltage is very good.
And in case of a catastrophic input stage failure the user
has much more fun:
I_fail=24V/10k = 2.4 mA ... enough to power a LED.
Perhaps this way the user gets enlightenment ;-)
...
> > I have done a few experiments with my comEEG and have got about 50mVp-p
> > 50Hz common mode voltage 1m away from power lines and
> > 350mVp-p with one foot stepping on an (isolated) 3-wire 230V
> > power line. (all without DRL)
>
> Joerg I really like the fact that you actually hooked up some
> electrodes and looked at the electrostatic pickup from the
> environment. Good work!
>
> This may not apply here, but the research that I was doing involved
> looking at the 40 Hz Gamma signals. This meant that I could
> not "filter" the signal.
I am against filtering (hi Q notch filter) too.
> I was able to get the 60 Hz "pickup" to a few microvolts using
> shielding techniques. Unfortunately the "infinite" common mode
> adjustment helped very little.
The CMRR in the datasheet of the input OPamp is normally not the
problem, because other factors are much mor limiting (e.g. low
common mode impedance).
> A real disappointment. I think you guys are spending way too much time
> simulating common mode rejection. Build something!
That is not alway the solution.
e.g. my current DRL stage (G=1000) oscillates at some 100kHz.
I have experimented with little capacitors but could not stop
the oscillations.
Now I will rethink the design in a simulation ;-)
Regards,
Joerg
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST