From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2002-01-30 23:34:41
Hi Jim and Joerg,
I must have missed this question from Joerg. Sorry.... :-p
> What do you think about the AD620 input current noise
> (especially at low frequencies) relating to the high-impedance
> high-pass network C4,R4 and C5,R5 ?
>
> I have used a similar (HP with asymmetric bias path) network
> with INA114 and got disastrous results (many uV of low frequency
> noise)
I honestly don't know... your filter had a big resistor in series
with the capacitor, perhaps that was the culprit? Have you tried
removing the 10Mohm resistor in series with the 100nF capacitor?
Jim wrote:
> So by inspecting the BrainMaster input stage, I guess at some low
> frequency the source resistance will be 10 Meg as the Xc of the
> 0.01 mf capacitor increases with lower frequencies. I wonder if
> you could try to simulate that stage again using larger capacitor
> values. I would be interested to see what capacitor value will
> give "good" noise down to 0.1 Hz? Somewhere I have the schematic
> for the original Mind Mirror. I think they used a 10 or 100 mf
> capacitors feeding into a discrete transistor for the lowest
> possible noise.
Jim, are you're talking about Joergs asymmetric filter?
Anyway, I did some calculations.
A network consisting of a 10Mohm resistor in parallel with a big
capacitor has these impedances at 0.1Hz:
C = 100uF => Z = 16Kohms
C = 270uF => Z = 6Kohms
C = 470uF => Z = 3.3Kohms
At 0.1Hz the resistance has very little influence. 10Mohms or no
resistor at all, give approximately the same result, meaning this
calculation applies to the BM-input as well.
So with a modern instrumentation amplifier the noise above 0.1Hz
should be negligible for C > 270uF.
The inevitable noise below 0.1Hz, caused by the resistor can be
removed by a digital filter, if you want the "raw" signal to look
more pleasing to the eye. For pure frequency anlysis, I don't see a
need for it though.
Joerg, could you try it out with your assymmetric filter, replacing
the 100nF caps with 270uF?
Jim, I hope you find the schematics for the mind mirror, I'd very
much like to see them. Not just the input stage, but the whole thing,
if possible. :-) I'm interested in buying the book by Max Cade as
well. Do you know where one can get it these days?
Regards,
Andreas
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