Re: [buildcheapeeg] Re: High resolution EEG schematic

From: Joerg Hansmann (info_at_jhansmann.de)
Date: 2002-03-09 00:07:58


Hi,

----- Original Message -----
From: Sar Saloth <sarsaloth_at_yahoo.com>
To: <buildcheapeeg_at_yahoogroups.com>
Sent: Friday, March 08, 2002 9:26 PM
Subject: Re: [buildcheapeeg] Re: High resolution EEG schematic

> I have tested that circuit but with a different OpAmp. Usually I do
> something like connect a capacitor or resistor to a function generator and
> use that as variable portion of the load.
>
> I do believe that you might have had it stable in your configuration, but
> then we had better use the identical components.

ACK.

> I do have the equipment
> to do such a test if you want me to test a particular combination. I don't
> mind doing that and sending you any waveforms.

That is nice !

I have put the schematic into the attachment ("modEEG_virtual_gnd.png")

The opamp should either be TLC272 or TLC277. AFAIK the only difference
is the price and the offset voltage.

> As I said before, the same circuit (although with Opamps that really DO
> make it unstable) has been in production for 15 years with very people
> noticing a problem. I can't tell you what product it was, but many have
> been made. Nevertheless, I can see some low level noise that shouldn't be
> there, and when I shock it it does indeed oscillate until powered down.

Looks like a very low phase margin.

> I
> guess my point is that just because people don't notice a problem doesn't
> mean that everything is OK.

ACK.

> I apologize, this is rather nitpicking, and
> with your explanation below I understand better. It is rather dumb to
> compare the TLC27L4 with the LT1114.

Even the TLC272 and the LT1114 perform quite different (in simulation)
with everything else unchanged.

> A similar circuit is working quite well in the comEEG prototype.
> >However I must admit, that the testing level is quite low
> >(on the oscilloscope I could not see oscillations).
> >To be sure a more thorough test should be done with a function
> >generator that injects a square wave into the control loop.
> >
> >(any volunteers here with the right equipment ?)
> >
> >IIRR the important trick in the virtual ground circuit was the
> >ESR of the tanatalum C at the output of the op-amp. An ESR of about
> >500mOhms provided a negative phase shift of about 90 degrees at the
> >0dB point in the bode plot of the loop gain, resulting in a total
> >phase margin of about 90 degrees (what is very stable)
>
> I didn't realize the ESR was so huge!

Can you provide some realistic ESR s for different C types and
values ?
I thought 500 mOhms were quite normal for a 47uF 10V tantalum type.

> That could make a big difference,
> especially for an OpAmp that is designed for some level of capacitive
> load. Relying on such ESR is definitely component (mostly OpAmp) and
> capacitor specific, and possibly also supply voltage and slightly dependant
> on the actual output voltage, so it would be necessary to verify the
> stability with the final configuration.

ACK.
However it is a "problem" common to LDOs and ground splitters
(e.g. TLE2426) too (as I have posted earlier).

> I just suggested the additions of the three passives because it is much
> easier to take them out than to put them in after the board is built.

In the LTSPICE simulation the addon did not perform very well ;-)
(Neither did the LT1114)

> Also, even if you are operating within the Cload (linear-tech's terms)
> OpAmps you will still have significant overshoot,

That is only true for not ESR compensated circuits with low phase
margin. About 90 degrees of phase margin do show very little overshoot.

> which would increase any
> noise on the virtual ground reference.

Yes.

> Those curves are done for
> capacitances without the ESR though, I suspect.

ACK.

> It was interesting that
> the LT1114 data sheet shows up to about 60% to 80% overshoot with 10nF
> load. Too bad they don't provide more data for loading the outputs.
>
> About your analysis - sorry for being obtuse, what did you use to get the
> 90 degrees from at the 0dB crossing? Does that include the op amp output
> impedance?

Yes. (At least I think so. To be sure I have to check the manual for the
TI provided spice models)

> This is getting me thinking, if the OpAmp output has a part in
> it, that might explain why the same circuit with an LTC27L4 OpAmp seemed
> stable until there was a significant perturbation - enough to cause the
> gain to drop due to loading the amplifier, which is when it went into
> oscillation.

If you provide the LTC27L4 spice model I could do some simulations.

> Another mystery solved ( to MY satisfaction anyway) !! I
> love this group!

:-)

> Anyway, I recommend putting the parts on the board just in case. If you
> want me to test anything, I can pay for shipping.

Thanks. The comEEG is only an obsolete prototype and not worth
any more expenditure. I think the testing should better be done
with the recent modularEEG amplifier.
(I have just bought the parts and will produce the prototype pcb
the next days)

> In the next two weeks I have to spend time sourcing parts, so I will look
> at these OpAmps more. Which OpAmp was used in the original design in the
> COmEEG?

INA114 and OPA2604AP, however as mentioned before this design is
obsolete now
and replaced by the recent "modEEGamp1_xxstd.*"
(you can find it on www.jhansmann.de in the eeg section)

The recent design uses INA114BP and TLC277 or TLC272.

Regards,

Joerg



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