From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2002-03-08 05:41:04
--- In buildcheapeeg_at_yahoogroups.com, Sar Saloth <sarsaloth_at_yahoo.com> wrote:
> At the risk of sound like a town-hall-committee wannabe,
> I would like to make some comments on the schematic.
> There hasn't been much of that nonsense here, so I will
> supply some. As can be seen below, I learned something
> trying to critique the circuit, but left the comments in
> for people's amusement.
>
> Has the virtual ground circuit been tested? My concern
> is that such a design can be unstable.
Hi Sar,
all comments are welcome.
To be honest, the control theory course was something I just took and
forgot. :-p This is Joerg Hansmanns design and he has done thorough
SPICE simulations (and real testing? Joerg?) that shows it is OK,
though not with the opamp I'm using. Personally, I raised the same
question as you do a few months back and maybe I should do
simulations of my own. Unfortunately, I'm swamped in work on other
parts of this OpenEEG project, so I'll have to ask you to show me the
hard numbers. ;o)
Feel free to download SwitcherCad III from Linear Technologies
(http://www.linear.com) and play with the virtual ground design.
(Just keep in mind and correct for, that real capacitors and
resistors are not pure capacitances and resistances.) You'll have the
full product range of Linear Technologies to try, including the C-
load types.
> TI makes a part that is a "virtual ground rail", one version of
> which is a constant 2.5V or so above the negative rail and the
> other part is a rail-splitter. I have never used either of them.
> They were supposed to be around a dollar and they are designed for
> this sort of application.
Yup it is a nice circuit. Unfortunately, the wideband noise figure
indicates it is suitable for 12-bit applications. Correct me if I'm
wrong, but any high frequency noise on the virtual ground will be
injected directly into the signal path as common mode noise.
> What is the reason for not using commercial instrumentation
> amplifiers? TI has a new low voltage single-supply CMOS InAmp
> that uses very little power (probably only in SMT) They send
> free samples.
It is because the ADC uses differential inputs, so the signal path
from electrodes to converter can be kept differential. The other
designs use regular in-amps (LT1167 and INA114).
Regards,
Andreas
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