From: Andreas Robinson (sleeper75se_at_yahoo.se)
Date: 2002-02-01 01:05:01
--- In buildcheapeeg_at_yahoogroups.com, "Joerg Hansmann"
<info_at_jhansmann.de> wrote:
Hi Joerg,
> The 10Mohm has not been in series with the 100nF C.
> It was the R-part of a highpass filter. So it can
> not be removed.
Oops. I meant "in parallel" of course... when the
capacitor shuts of at low frequencies, you basically
have a 1Mohm resistor in series with the input,
causing a lot of noise, perhaps because it is a real
resistor, not a capacitor with resistance. My thought
was that with a bigger capacitor, you could "push"
that noise down below 0.1Hz, where it is out of the
way.
> With these _very_ big capacitors Z would be OK and
> noise also, however for these values you would need
> to use electroytic (or tantalum) type Cs and they
What? Are there no 100uF ceramics? ;o)
[snip]
> It would not work because of the above reasons.
Ok, just a thought.
By the way, why don't you use an integrator plugged
into the REF-input? That would allow for higher
instrumentation amplifier gain and unchanged offset
handling capabilities.
> An exact solution would be to integrate the 1/f
> noise (that merges with the white noise somewhere
> above 100Hz) shaped by the HP-filter over the
> amplifier bandwidth (e.g. 0.16 Hz .. 75 Hz for
> modularEEG or something else for the BM).
> I have tried this with SPICE but was not successful
> until now (Johnson noise is no problem, but how do I
> get/simulate 1/f noise ???
> I have used a current source forward driving a diode
> but can not see any 1/f noise ... :-(
I surfed the net a bit... The magic diode parameters
are KF and AF. I calculated KF based on the
current-noise properties of the LT1167 and made a
diode model with only those two parameters. It appears
to work.
In short: Add a diode to the schematic, name it
whatever you want, such as NOISESRC, then write a
SPICE-directive that says
.model NOISESRC D(KF=1.31e-17 (or whatever), AF=1)
The calculation and spice simulation is attached.
I had to play around a bit with the DC-current of I1
(60000 amperes, how does that sound? :-) to get the
right scale (1V/A), but you can probably improve from
there on. Feel free to compare with the LT1167
datasheet:
http://www.linear.com/pdf/1167f.pdf
page 9.
> @ Andreas:
> In another posting you have shown your excellent
> skills in solving noise equations. Perhaps you can
> comment on this...
Thanks for your faith in my abilities. :-) It's just
"plug in equation A into equation B and give it to the
supercomputer (My HP-48 calculator)".
Regards,
Andreas
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