From: Andreas Robinson (sleeper75se_at_yahoo.se)
Date: 2002-01-08 20:08:38
--- In buildcheapeeg_at_yahoogroups.com, "Jim Meissner"
<jpmeissner_at_mindspring.com> wrote:
>
> > One might call it a 2.5-layer board.
> Yes that works. But that might be an overkill.
Think of a
> separate preamp for all the fancy stuff. Then the
other board is
> a "normal" project.
Hi Jim,
2.5-layer PCB's is my "normal" way of building boards.
:-) I've developed a simple technique for aligning the
two layers when I do the UV-light transfer, so it is
almost as easy as making single sided boards.
In another thread you wrote:
> My sampling rate was 130 Hz and I was able to send 5
bytes at 9600
> Baud. That generated megabytes of data in a few
minutes. Why are
> you sampling at 1KHz?
Well, it reduces the analog filter order and lets me
use a relatively high cutoff frequency. That reduces
the falloff and group-delay change of the Bessel
filter in the 0-60Hz range. On the PC, I can filter
and downsample the signal to perhaps 128 or 256Hz.
I'll use an IIR filter, for this, because they have
very short group delay (compared to FIR's) and they
only require a few multiplications per incoming
sample.
I've uploaded a new input stage, based on the LT1167
and your suggestions to the files-section. Please take
a look at it and let me know what needs to be
improved.
I have also completed the layout for it. I haven't
bothered to name all parts, but you can get the
general feel of where the power lines go and how the
preamp stage looks. I managed to fit one channel
preamp + filter into 81x33mm (3.2 x 1.3 inches).
Both files are here:
http://groups.yahoo.com/group/buildcheapeeg/files/hardware%20design/PDF_and_PNG_schematics/
Some general notes:
* The filter has no longer any gain - that saves a
couple of resistors but costs an extra opamp. Another
benefit is that you can easily change the gain without
redesigning the whole filter.
* The highpass order is now 2, with a cutoff freqency
of 0.3Hz. That should allow for other electrode
materials with high offsets, such as gold and tin. If
someone wants DC response - he/she can just replace
two caps with shorts.
* I estimate the cost for one channel without DRL will
be €20 with high-quality filter-amplifiers, so a 2
channel system with DRL and a Tiny-AVR might go for
€60 in parts.
* I will design the DRL on a separate board, after the
regular preamp is working properly.
/Andreas
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST