From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-01-20 04:59:24
Dear Joerg:
I hope you don't think that I am trying to argue with you. I am sharing similar experiences I have had in the past and how I solved them. It is meant to be helpful.
I believe that having all the grounds, digital, analog, shield, and case atthe same potential ( Star Ground ), is a must. I will go to great length to make that possible. ( Including separate batteries if needed )
When a system has a large gain such as looking at 0.1 microvolts, it is easy get positive feedback loops and oscillations. Ground loops and power supply rails are the main source of feedback. Modern op amps with their great power supply rejection ratios have made us complacent. I have designed with tubes and discrete transistors and learned the hard way.
The "redesign" should not be that difficult. Take "most" of the V/2 lines and put them on ground. Un-ground a few op amp power supply pins and run to the - 3 volt buss. Put a low dropout regulator from the + 5 Volt digitalpower to generated + 3 volts. Run a 7660 or such from the + 3 volt analog power to generate a - 3volts.
I do not know exactly which version you are working on so this is a generaltype comment. If could tell me which file to look at, I would be happy tobe more precise.
++++++++++++++++++++++++++
On the other hand, you state that you do not like negative voltages. Even though I disagree with your viewpoint, I will make some suggestion if you want to stay with the present design.
First you have to have a heavily bypassed + 5 volt digital power buss. It takes several types of capacitors to do that. I notice that you use a 47 uf and a 100 nf. What I like to see is a 100 uf high frequency electrolytic, with a 10 uf tantalum and several 100 nf caps. As you know the physical placement and lead length and a good ground plane is important. Then I would look at the split voltage of 2.5 volts or V/2 from a different perspective. Try to make the impedance from this V/2 buss to the ground plane as low as possible. Put a 100 uf, 10 uf, 100 nf combination from the V/2 to theground plane. This of course is a terrible load for the centering op amp or a "rail splitter". I does not have enough drive current to move such a high capacitive load and will oscillate. Try to use a low drop out regulator chip to drive the + 2.5 volt buss from the + 5 volts. That way it thinks it is a 2.5 volt power supply and will be happy.
Whether to regulate the + analog power supply line is another matter. I think at least there should be a resistor and another capacitor network just for the analog + volt buss. You do not want digital noise on the analog power buss.
Hopefully this is of some help.
Juergen P. (Jim) Meissner
Check out my Website at www.MeissnerResearch.com
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----- Original Message -----
From: Joerg Hansmann
To: buildcheapeeg_at_yahoogroups.com
Sent: Saturday, January 19, 2002 9:58 AM
Subject: Re: [buildcheapeeg] virtual ground problem
Dear Jim:
----- Original Message -----
From: Jim Meissner <jpmeissner_at_mindspring.com>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Friday, January 18, 2002 5:19 AM
Subject: Re: [buildcheapeeg] virtual ground problem
>Dear Joerg:
>
>Let me restate my objection to the "floating" ground concept. OK? Next.
>
>In hindsight I have wasted a lot of time "solving" this problem in my designs.
>The solution that worked to some degree was to make the bottom a fixed voltage and
>let the top one move.
What I need is 2V above the bottom (= digital ground) and on top of that
a fixed 3V source.
Why should I let the 3V unstabilized ?
That would not be good for the At90S4433 processor.
>That way you could have a large capacitor "maintaining" the
>voltage and not require the centering op amp to "change" the voltage anddrive
>into a large capacitor. It is very difficult to have high loop gain driving into
>a capacitor.
A "normal" opamp (e.g TLC272) has difficulties driving a more or less large
capacitor. The reason is, that in the open loop bode plot the large bypass capacitor
forms a lowpass (or "pole") with the opamp output resistance. This pole must _not_
come too near (about f * 10^4 distance) to the first pole of the opamp, else the
phase reserve decreases to a few degrees
(180 degrees - 2x nearby 90 degrees per lowpass) and we have ringing and gain or
impedance peaking.
The "solution" is to use special opamps/circuits which first pole is far away
from the bypass C / output resistance pole:
e.g. the virtual ground rail splitter TLE2426
or even the well known 7805 regulator (internally a reference, an opamp and a power
transistor) that _needs_ the bypass C in order to be stable.
>Ultimately I found it much, much easier to use a DC/DC converter to generate a +
>and - buss.
I think, this is a very good solution, if much power is needed.
>Just to drive a few op amps, a 7660 will invert the + 5 volts into a
>tracking - 5 volt supply.
I would not be too happy with negative voltages, but that of course is
a question of the viewpoint.
To adapt this concept to the modularEEG, I then would need to down-regulate
the -5V to -2V and the +5V to +3 V. The -2 V would have to be derived somehow
from the 4V reference for the ADC in the At90s4433.
All in all it could be done somehow, but the expenditure would be high
and all in all it would be nearly a complete redesign.
>The other advantage is that now the digital ground and
>the signal ground and shield can all be at chassis ground.
>
>Unless you like a real challenge, give it up
what means redesign
>and get on with the project. I think
>you are a very clever designer. Let's spend the effort in getting a working EEG.
The prototype _is_ working. However it has some design flaws.
IMO these flaws should be corrected without changing too much.
>The customer does not care what clever designs are in the box as long asit works
>well.
That is right.
>
>Any progress with the high voltage isolated DC/DC power supply?
Yes. See other posting.
Regards,
Joerg
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