From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2002-04-21 15:30:52
--- 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.
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, 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.
Regards,
Andreas
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:43 BST