From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-01-04 20:46:25
Hi Andreas,
----- Original Message -----
From: sleeper75se <sleeper75se_at_yahoo.se>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Wednesday, January 02, 2002 10:09 PM
Subject: [buildcheapeeg] Re: Input protection
> Hi Joerg,
>
> --- In buildcheapeeg_at_yahoogroups.com, "Joerg Hansmann" <info_at_jhansmann.de> wrote:
> > Lowest possible operating voltage of the used OPs.
> > Are they all rail-to rail ? (Or at least near to that)
>
> Ah, now I understand. Ok, the datasheet for LT1012 says this:
>
> Minimum supply voltage: +/-1.2 volts
> CM-range: typically V+ - 0.9V and V- + 0.9V
> So powering it at +/- 2.5V would give you a common mode range of +/-
> 1.6V.
That should be OK.
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)
> CMRR is 40dB at 1MHz, falling about 20dB per decade, by the way.
> It is not rail-to-rail, at +/-15V supply voltage the swing is
> characterized to typically +/- 14V. I don't know how that scales, but
> a wild guess is +/- 1.5V at 2.5V supply voltage.
Should be OK if offset voltages are cancelled / or gain
of each stage is not too high.
> > 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.
>
> Yes, you are right, it does not attenuate the HF common mode signals,
> but one could argue that those are not amplified either...
But the HF gets demodulated at non-linear components in the OP and
thereby adds a seemingly unpredictable offset voltage that
really is music from the next AM-transmitter.
> Of course, better safe than sorry, but isn't it hard to get the the
> ground/shield quiet enough for what is essentially a HF short-circuit
> to the most sensitive, high-impedance part of the system?
The ground plane should have really low-impedance. Alternatively
or additionally the Cs could be connected very close to the
electrode connector and a metal casing could be used (at least
for the input stage because it could corrupt isolation if it
extends over the optocoupler stage).
> Hmm, I think we have reached the point where we need to build and
> test.
Experiments are always good...
> > 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 ...
>
> Unfortunately I was unable to find a model for BAT48 and the data
> sheet only characterized the reverse leakage at 10V and up... (2uA)
I have simulated a BAT 54 (30V, 0.3A) with switchercad
and can`t believe how bad the leak current/impedance is compared with
a npn (e.g 2n3391)
> > > 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.
>
> A differential amplifier built from two N-channel MOSFETS measuring
> the voltage drop across a resistor, would that work? They are nice
> and quiet AFAIK.
And have ESD sensitve gates or ... protection diodes at their gates.
> Since the amplifier would be discrete, we could
> limit the drain-source current using large resistors (100K+) and keep
> the user safe.
Why not construct the 1st amplifier stage with discrete components
to get hi-impedance and low noise ?
(However do not know enough about analog design to do that...)
> Noise would not be that important if the system
> averages the measurement over perhaps 100ms.
I thought more of the additional noise from the protection circuit that could
affect the EEG-measurement...
> A high overcurrent
Through the test subject ?
> would
> trigger the shut-off circuitry faster than an overcurrent that is
> only a little bit over the limit.
I have no idea, what IEC601-1 says how long the
current limits may be exceeded.
Regards,
Joerg
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