From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-01-09 12:31:31
Dear Andreas:
I have looked at your schematic and there are some errors. Also I will give you my opinion that have to do with philosophy rather than saying it is wrong. Check this out with the chief engineer Joerg Hansmann.
Error: Op amp U2A is not correct. R3, R4 should go to the (+) input. C4,C5, C6, need to be two 10mf tantalum capacitors back to back. U3C ( running from +/- 3 volts ) will not be able to drive the A/D converter to full scale. That op amp section needs to be a rail to rail amp running from + 5 volts and ground and located with the A/D section.
Opinion: I see no reason to use Q1 and Q2. The LT1167 internal protection with 1.6 K for R1 and R2 will be good enough. There cannot be a high energy voltage between the patient ground and the electrodes. He would be dead.There could be a common mode electrostatic high voltage ( the whole patient is at a high voltage), and that protection would be handled by the opto coupler. I also recommend that you stay with the capacitor values suggested by Linear Tech in Fig. 5. C1, C2 = 100pf, C3 = 10pf. ( In my design I used 270pf for C1,C2 )
Opinion: If you are planning to use an active patient ground reference youshould look at Linear Tech Fig. 8 and see how they do it. But, my opinionis to start out simple and use a passive patient ground. Connect J1 pin 2to chassis ground and do not use D1, D2, R12, R5, U2.
Opinion: Use only one high pass pole. You are using 3 it looks like. From the FFT standpoint it would be better to DC couple the signal. You will get incorrect brain wave frequency data in the Delta frequency region. Probably C6 should be the only high pass RC network. Eliminate U2B, C5, C4, R9. Ground U1 pin 5.
Opinion: The power supply RC filter could be much larger. R24, R25 is now10 ohms. It could be 100 ohms or more.
Opinion: The U1 gain could be set much higher and U3 lower. Also U1 has half the noise at a G = 100 vs. G = 10.
Opinion: The +/- 3 volt supply for the op amps should be higher to give some headroom above the minimum operating point and considering battery voltage variation and discharge.
Andreas, I know that it is your intention to build a Mercedes, but I am suggesting to start with a Volkswagen and see what the road is like and then make the "appropriate" improvements.
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.
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