From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-01-18 04:19:50
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 fixedvoltage and let the top one move. That way you could have a large capacitor "maintaining" the voltage and not require the centering op amp to "change" the voltage and drive into a large capacitor. It is very difficult to have high loop gain driving into a capacitor.
Ultimately I found it much, much easier to use a DC/DC converter to generate a + and - buss. Just to drive a few op amps, a 7660 will invert the + 5 volts into a tracking - 5 volt supply. The other advantage is that now thedigital ground and the signal ground and shield can all be at chassis ground.
Unless you like a real challenge, give it up 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 customer does not care what clever designs are in the box as long as it works well.
Any progress with the high voltage isolated DC/DC power supply?
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: Joerg Hansmann
To: buildcheapeeg_at_yahoogroups.com
Sent: Thursday, January 17, 2002 8:05 PM
Subject: [buildcheapeeg] virtual ground problem
Hi,
I have done simulations with SWCAD III on the virtual ground circuit
(that buffers Vref/2) of the modularEEG.
Originally the circuit was a TLC272 used as a follower (unity gain).
To get low HF output impedance I have bypassed the output to V-.
The problem however is, that compensated OP-amps (like the TLC272)
with capacitvely bypassed outputs show gain peaking and ringing, because
almost no phase reserve exists (about 1 degree).
The circuit with C5,R5, R6 is somewhat better than the original unity gain follower,
but still exhibits resonance at about 8.5 kHz.
The result seems quite usable to me, however it would obviously be better, if the
phase reserve were greater. Any ideas here how to achieve this without
replacing U1 with an uncompensated type ?
I could imagine that some type of dominant pole compensation
(with a matched zero) should work but have no idea how to realize this.
In the attached graphics a star formation consisting of 3 loads (Rl1..3, e.g. ICs)
connected to the virtual ground (Vref/2) is shown.
At Load1 a current is injected (noise, spikes, etc.) and at Load3 the resulting
voltage is recorded.
Regards,
Joerg
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:36 BST