From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-04-21 22:48:53
Hi Andreas,
----- Original Message -----
From: sleeper75se <sleeper75se_at_yahoo.se>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Sunday, April 21, 2002 4:30 PM
Subject: [buildcheapeeg] Re: TinyEEG update
> --- In buildcheapeeg_at_yahoogroups.com, "Joerg Hansmann" <info_at_jhansmann.de> wrote:
>
> Hi Joerg,
>
> > > Well... since I have this power-rail feedback problem,
> >
> > Under what test conditions does this problem occurr ?
>
> Under realistic conditions I think. Jim Meissner gave me a very
> clever schematic for a microvolt attenuator (Thanks again Jim).
> It consists of two resistive voltage dividers connected in series,
> and the resistances are chosen so that the input-to-output resistance
> is about 10kOhms. The resistors are placed in a metal box, for
> shielding.
>
> So, the test setup was this:
>
> Function generator (1V sinewave) => attenuator (10uV sinewave) => EEG
> amplifier and filter - one input grounded => oscilloscope.
The function generator is normally grounded and the oscilloscope
too. Are you sure, you did not built a ground loop ?
Does the oscillation continue, if you disconnect the function generator ?
> I used coaxial cable wherever possible so the signal was shielded all
> the way from the function generator to the inputs of the EEG
> amplifier.
>
> The amplitude of the unwanted oscillation is around 10uV referred to
> input (25mV measured on the scope) and very low frequency, less than
> 1Hz. It is not a symmetrical sine wave, but more sawtooth-like. It
> was really hard to notice at first, since there was 10-20 uV 50Hz
> interference (RTI) imposed on the signal,
How can that be with all the shielding ?
> and the oscilloscope tended
> to trig on that rather than my test signal. I saw it when I disabled
> the trig and let each sweep cover perhaps two seconds. Easy to do
> with a digital scope...
>
> Fortunately a 10 ohm + 1000uF RC filter on each rail, between stages
> should be sufficient. We'll see.
Good luck !
Regards,
Joerg
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