From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2002-01-02 21:09:25
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.
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.
> 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...
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?
Hmm, I think we have reached the point where we need to build and
test.
> 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)
>
> Do such things like shottky transistors exist ?
> (Like in the 74LS- TTL-series)
I have never heard of that... Hmm, question is if they wouldn't leak
as much as a diode. Part of the regular bipolar transistors low
leakage (and poor user-protection) could depend on the high turn-on
voltage. (0.7V)
> > 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.
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. Since the amplifier would be discrete, we could
limit the drain-source current using large resistors (100K+) and keep
the user safe. Noise would not be that important if the system
averages the measurement over perhaps 100ms. A high overcurrent would
trigger the shut-off circuitry faster than an overcurrent that is
only a little bit over the limit.
Hmm, I think I'll read up on MOSFETS...
> > 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 read the addendum. Just ignore my suggesting Simetrix. :-)
I have not installed SwitcherCAD yet, but an unlimited version is
always better than any limited one, if it is user-friendly enough.
> > I made a little practical experiment the other day:
[snip]
> > 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.
Perhaps I was measuring the resistance of the saline solution on the
surface of the skin ...
Anyway, when the electrodes are finished, I'll hook myself up to a
prototype EEG and do a real test with everything done properly.
Regards,
Andreas
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST