From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-01-02 01:20:39
Hi Andreas,
----- Original Message -----
From: Andreas Robinson <sleeper75se_at_yahoo.se>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Tuesday, January 01, 2002 4:30 PM
Subject: [buildcheapeeg] Re: Input protection
> Hi Joerg,
...
> Well, the only thing I can see is limited by low
> voltage is common mode range, but that has nothing to
> do with the protection. Can you think of anything
> else?
Lowest possible operating voltage of the used OPs.
Are they all rail-to rail ? (Or at least near to that)
...
> > For what purpose are the 1nF C's C1..3 ?
> > For shunting HF they must be connected to a low-
> > impedance node (what they are seemingly not)
>
> <confused> They don't attenuate HF?
Only in differential mode as far as I can see.
However common mode rejection of OPs decreases at high
frequencies. So HF should also be shunted for common mode.
> My intention here
> was to let the capacitor act like a short for high
> frequencies between a channel and the reference. It
> looks good on paper/in simulation.
Also for common mode HF signals ?
...
> Please consider using transistors rather than diodes.
> See the attached schematic where I've replaced the
> diodes with transistors.
> The transistors don't not protect the user very well
A forward voltage of 0.7 V together with the 15k R
would in worst case exceed 50uA. On the other hand
I do not like to increase the 15k significantly
(in combination with INA114).
> due to their higher turn-on voltage, but the diodes,
> while an effective protection, leak far too much.
>
> BAR42 schottky diodes (Vf = 0.4V supplied with
> Simetrix, that's why I used them) leaked 1nA with a
> 1mV signal. The transistors leak only a fraction of
> that, around 2pA. It gets worse when you add an
> offset. At 100mV offset the diodes let 100nA through
> while the transistors stay at a lower 20pA. Of course,
> all these calculations depend somewhat on what
> capacitors are used...
Oops, I did not realize that shottky diodes were so bad.
Are BAT48 diodes(max 40V, 0.35 A) comparable with BAR42
(max 30V,0.1 A)?
I fear they are even worse ...
Do such things like shottky transistors exist ?
(Like in the 74LS- TTL-series)
> I'm beginning to think we would need to measure the
> current through the electrodes and have an active
> emergency-shutoff...
I have had the same idea, but could not find a
way to measure the error-condition without
producing noise, degrading impedance or adding new
risk by the protection circuit.
> That is, if we should still aim for FDA/IEC
> compliance?
At least IEC, because it is easier to comply.
> I'm leaning towards just putting something
> small and simple together now, so that people can get
> started writing software
For writing SW the digital part would be enough.
> as soon as possible. FDA can
> perhaps wait a bit... and the paranoid can always
> check the amplifiers with a multimeter before
> connecting themselves.
>
> > BTW: Do you have an idea how the PSPICE AMB
> > equations for OP-amps looks like ? (gain and
> > three poles would be enough ...)
>
> Unfortunately no.
> I'm guessing here that your spice-simulator lacks
> built-in support for opamps?
It is restricted (freeware version) to 3 real op-amps
simulated on transistor level or 50 parts over all.
> You have previously
> mentioned that you were forced to simulate opamps
> using voltage controlled voltage sources.
Yes, it is a more or less accurate work-around.
> In that case, take a look at Simetrix:
> http://www.newburytech.co.uk/
>
> The user interface is a bit different, and the free
> version limits circuit size, but other than that, it
> is very good.
I will have a look at it. (At the moment I am trying switchercad,
a free and _unlimited_ PSPICE. See addendum for more)
...
> I made a little practical experiment the other day:
>
> I've built some electrodes of 18mm silver discs, and
> using two of them I measured the impedance on the
> lower arm.
>
> First it was cleaned with alcohol and salt water was
> poured onto it.
A kind of sponge or cellulose tissue that is soaked with the
saline would be better I assume.
> The electrodes (non chloridized, also
> cleaned with alcohol) were applied and pressed hard
> onto the skin. The resistance was measured with a
> multimeter.
Normal multimeters use DC current to measure resistance.
This quickly produces a galvanic reaction (like loading a
battery) and falsifies the reading.
> The results: 160Kohms when they were about 10 cm apart
> and 24Kohm when they were 1 cm apart.
Hmmm. the distance should not matter, because the transitional
resistance of the skin to the body internal normally is much higher
than the resistance of the internal.
> I would have gotten lower values
Perhaps you should use 10/20 paste wax...
Regards,
Joerg
BTW: I am just testing this here:
(could be a good alternative to my limited microsim eval version)
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST