From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-01-08 15:34:57
Dear Andreas:
Take another look at the EEG preamp stage that I posted on my website. This is a shielded box that is worn on a belt. The output cable can be run 10to 100 feet without degradation to the A/D converter.
Think about your preamp in those terms. It could be an inch away, but think of managing your power and signals as though it is 100 feet away.
> The solution was to add resistors between stages of bypass
> capacitors to make R-C filters.
The op amps have a 100 nf cap from (+) power bus to ground, and a 100 nf from (-) power bus to ground. As the power enters the board, there are 10 mftantalums from the power bus to ground. This allows me to put a resistor between the preamp and the power source. The larger this resistor the better the RC filter. Too high a value and too much voltage is lost and voltage regulation-stability becomes a problem.
> > Won't breadboarding cause a lot of trouble in itself, like high
> > parasitic capacitances?
> Just the opposite. PC board will have much higher (but controlled)
> capacitance than point to point wiring.
> Ok. Well, I am planning to put a more or less unbroken ground plane
> on the top side and run all signals on the bottom. That creates a lot
> of capacitance...and it is not exactly a star ground, so it is
> perhaps not a good idea?
Look at the schematic and notice that I purposely added a 10 nf capacitor from the signal output line to ground! Also notice the op amp configurationnecessary to be able to do that. The purpose was to reduce the impedance of the line to reduce noise pickup. Capacitance in the right place is good.
> Also, whenever two signals intersect I will
> use a piece of wire so that I don't need to break the ground plane.
> One might call it a 2.5-layer board.
Yes that works. But that might be an overkill. Think of a separate preampfor all the fancy stuff. Then the other board is a "normal" project.
> You will need some filter ahead of the A/D to get rid of aliases.
> I've designed the filter for just that. If the sample rate is 500Hz
> or more, there is no theoretical possibility of aliasing up to 100Hz,
> with a 10-bit ADC. 1kHz samplerate is sufficient for 16-bit precision.
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?
Juergen P. (Jim) Meissner
Check out my Website at www.MeissnerResearch.com
Read about the benefits of the Brain State Synchronizer sounds for improving your life and health.
----- Original Message -----
From: sleeper75se
To: buildcheapeeg_at_yahoogroups.com
Sent: Tuesday, January 08, 2002 8:24 AM
Subject: [buildcheapeeg] Re: TinyEEG
--- In buildcheapeeg_at_yahoogroups.com, "Jim Meissner" <jpmeissner_at_mindspring.com> wrote:
> Dear Andreas:
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST