From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-02-02 02:30:21
Dear Andreas:
> Actually the ground-plane under the resistors is connected to the
> shield, not ground. On both sides. Bad idea?
> Ok, how about this solution: The 100pF capacitors connect to the
> shield instead. Then the shield driver's primary job would be to keep
> LF-noise (such as 50/60Hz hum) out, while the caps take care of the
> HF.
No, I would tie those to ground, both sides of the board to ground.
I have done the driven shield design where it is critical to reduce the input capacitance. Usually that required a center conductor, a driven shield and another shield around that going to ground, a Triaxial cable. In this case that is of no importance. In my preamp I had 270 pf additional capacitors to ground, so the 20 pf per foot of the cable was of no consequence.
> I could design a layout without shield driver and filter if you
> like...? It is not much work, I'll just rip out whatever is not
> needed.
I am trying warn you of pitfalls without squashing your creative juices. Step one is to build "something" and test. Then you will have an opportunity to determine whether this driven shield is a good idea or not. Also I amnot convinced that this fancy filter is needed unless it was designed to reduce the 50/60 Hz interference. My design used no filters and worked quite well.
Juergen P. (Jim) Meissner
Check out my Website at www.MeissnerResearch.com
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----- Original Message -----
From: sleeper75se
To: buildcheapeeg_at_yahoogroups.com
Sent: Thursday, January 31, 2002 8:46 PM
Subject: [buildcheapeeg] Re: TinyEEG - Input grounding
--- In buildcheapeeg_at_yahoogroups.com, "Jim Meissner" <jpmeissner_at_mindspring.com> wrote:
>The RF from the radio station and equipment switching glitches are
> not attenuated. The output impedance of the op amp is not low
> enough at these frequencies. In your case you even have a 220 ohm
> resistor in series to make matters even worse. The shields would
> actually drive noise into your groundplane under the input
> resistors and right into the input stage.
Actually the ground-plane under the resistors is connected to the
shield, not ground. On both sides. Bad idea?
> I am not impressed with simulations done on a computer.
> The real world does not care what you simulated.
Actually, I haven't simulated the shield driver. It's just touch and
go. :-)
Ok, how about this solution: The 100pF capacitors connect to the
shield instead. Then the shield driver's primary job would be to keep
LF-noise (such as 50/60Hz hum) out, while the caps take care of the
HF.
> If I were to use your board, and I certainly want to, I would first
> rip out all the driven shield components and tie J100 pin 3 to
> ground. C100 and C101 are already on ground. The rest looks good.
> (BTW I would not use the filter section either)
I could design a layout without shield driver and filter if you
like...? It is not much work, I'll just rip out whatever is not
needed.
>
> Could you make these little input boards first and test them? That
is by far the most important step.
> After that everything is much less critical.
I'm on it.
--- In buildcheapeeg_at_yahoogroups.com, "Jim Meissner" <jpmeissner_at_mindspring.com> wrote:
> I looked at the A/D converter section and I think it looks good
> enough the way it is to build and test.
>
> Full speed ahead. The testing will tell the story.
I just got the order confirmation from Digikey. It really is full
speed ahead. They obviously didn't notice that I wanted FedEx
International Economy, they ship it with FedEx International Priority
(2 days) instead. Shipping is *only* 48 bucks (notice the sarcasm).
Thing is, my thesis is due in a week, so I'll have to slow down the
EEG project for a while...ho hum.
Regards,
Andreas
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:37 BST