From: Jim Meissner (jpmeissner_at_mindspring.com)
Date: 2002-01-09 21:36:02
Dear Andreas:
> C1, C2 = 100pf, C3 = 10pf. ( In my
> design I used 270pf for C1,C2 )
> But the input resistors are 4.7K, not 1.6K. Won't that give similar
> results?
NO this is not an RC filter. You want to provide a very low impedance to the RF world. The lower impedance ( larger cap. ) the better. Too large a capacitor will affect the signal. The signal is affected by the RC time constant. In another design I used zero ohms and 10 nf caps. I was using a transmitter mounted on a baseball cap on top of the head. (worked very well)
> But, my opinion is to start out simple and use a passive
> patient ground. Connect J1 pin 2 to chassis ground and do not use
> D1, D2, R12, R5, U2.
> Maybe I was unclear, because U2A is the shield driver, not the DRL.
> Hmm, an expensive shield driver... ($2.20 for half a LT1112). A
> cheaper part like TL061 ($1) will do.
If look at the input stage I built you will find that I designed it with shield drivers! Sounded like a good idea. But, the real world strikes again. It did not work nearly as well as grounding the shields to chassis!
> Question 1: Should the shield driver have the same feedback network
> as in the Linear design, by the way?
As a shield driver your voltage follower was correct. BUT, see above.
> Question 2: Why have they split the gain resistance into three
> resistors in Fig 8, rather than just using two? Is it for better
> control of the gain?
That is a cute delta to Y transformation. That way one resistor Rg controls the LT1167 gain, you only have to change one resistor to change gain. The two 30 K's are really a 15K setting the gain of the LT1112.
> gain to 100 and it would still tolerate +/-
> 60mV offsets which is enough for Ag/AgCl electrodes.
My preamp gain was 100 and I do not recall having offset problems. I was not even aware of this 60 mv offset until I hear you and Joerg bouncing it around. Are you sure that it is not 60 microvolts? Also if both probes have a 60 mv offset the diff amp will see that as common mode and cancel it out. (BTW) Who in this group has "actually" worked with electrodes and can answer these questions? ( yaniv do you know? )
> Ok. As long as it can have a Porshe engine. :-) Your suggestions can
> save a few dollars though, and that is always good.
My suggestion are not related to saving money. I am pushing for working prototype, even a poorly working prototype. Right now all we have is theories bounced around, nothing practical.
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: Wednesday, January 09, 2002 12:02 PM
Subject: [buildcheapeeg] Re: New input stage
--- In buildcheapeeg_at_yahoogroups.com, "Jim Meissner" <jpmeissner_at_mindspring.com> wrote:
> 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.
Hi Jim,
opinions are good. Makes people think.
Joerg, you are welcome to comment. Please do.
> Error: Op amp U2A is not correct. R3, R4 should go to the (+)
> input.
Thanks for spotting that. I'll just flip the amplifier upside down.
> C4, C5, C6, need to be two 10mf tantalum capacitors back to back.
I was planning to use bipolar caps here. They are $0.37 each at
Digikey.
> 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.
Hmm, the biasing stage should in other words be somewhere else... ok,
I'll move it to the ADC board.
> 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.
I probably wont be using them, I'm having a hard time finding them or
a suitable replacement type.
> 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 )
But the input resistors are 4.7K, not 1.6K. Won't that give similar
results?
> Opinion: If you are planning to use an active patient ground
> reference you should look at Linear Tech Fig. 8 and see how they do
> it. But, my opinion is to start out simple and use a passive
> patient ground. Connect J1 pin 2 to chassis ground and do not use
> D1, D2, R12, R5, U2.
Maybe I was unclear, because U2A is the shield driver, not the DRL.
Hmm, an expensive shield driver... ($2.20 for half a LT1112). A
cheaper part like TL061 ($1) will do.
The DRL will be on another board, because it is an optional feature.
Question 1: Should the shield driver have the same feedback network
as in the Linear design, by the way?
Question 2: Why have they split the gain resistance into three
resistors in Fig 8, rather than just using two? Is it for better
control of the gain?
> 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.
I'd rather keep the AC-coupling. Disabling it is very easy. I can
test both options and see how they work.
> Opinion: The power supply RC filter could be much larger. R24,
> R25 is now 10 ohms. It could be 100 ohms or more.
Ok, no problems.
> 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.
That was a tradeoff for the highpass filter ... but you are right. I
think I can raise the gain to 100 and it would still tolerate +/-
60mV offsets which is enough for Ag/AgCl electrodes. But I need the
second HP-pole to achieve that. The third pole is just for the
biasing circuitry, which can be changed to DC-mode.
> 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.
I agree. I'm planning to use a separate battery or a 3-5V voltage
regulator for the microcontroller. Then it would be possible to use
any voltage in the analog section. However, the opamps can operate
down to +/-2.25 V and still give me a 2.5V swing - enough for the ADC.
> 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.
Ok. As long as it can have a Porshe engine. :-) Your suggestions can
save a few dollars though, and that is always good.
Back to the CAD program....
/Andreas
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