From: Sar Saloth (sarsaloth_at_yahoo.com)
Date: 2002-03-11 20:04:41
To all those bored by the subject -
Joerg was right so no need to read on.....
All else
Joerg,
This is one of the emails that I was writing at the same time as you (last
week). Just so you know I was not ignoring your answers, I am sending it
here. I was afraid our crossed emails were causing too much noise, so I
just held off until I saw that we had no more crossed emails.
old email I didn't send (it was crossed)...........................
>ACK.
I am very new to this news-group type stuff. What does ACK stand for? By
your usage it is more like [NACK] than [ACK], should I assume it is like
ARGH!, then it would make sense to me. (;.<>
> > 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.
Wouldn't it be a non-linearity if the size of the impulse was an issue? If
it was part of the linear response, couldn't it be set off even by a small
signal?
> > 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.
I meant that I didn't realize ESR values were so large. I have gone back
and found that I usually didn't use tantalums in those situations, so what
I thought was universally stable was only in my circuits.
I apologize for the false claim.
> > 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.
I think I understand now.
> > which would increase any
> > noise on the virtual ground reference.
>
>Yes.
>
>
> > Those curves are done for
> > capacitances without the ESR though, I suspect.
>
>ACK.
I think that is what you said (that the overshoot curves in the data sheet
were for capacitors without ESR or what you called ESR compensated
circuits). Maybe you were just on too great a roll with the 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.
Oops, I meant TLC27L4, almost identical model to your TLC272.
Anyway, thanks for the education. I am posting the circuits you posted,
but with my circuit "tuned" and with a load added to look at load regulation.
Your original circuit is superior here also! |:.( I guess the only
place my circuit would be better is if the circuit required too many
distributed ceramic capacitors so that your circuit would become
unstable. Is that right?
btw, the FilterCad UI is very nice. Is there a commercial full-featured
product with a similar UI?
Thanks for the education.
I have uploaded the tuned circuit and the responses....
http://groups.yahoo.com/group/buildcheapeeg/files/Virtual%20Ground/VirtualGnd-InputResponse.pdf
http://groups.yahoo.com/group/buildcheapeeg/files/Virtual%20Ground/VirtualGnd-LoadRegulation.pdf
http://groups.yahoo.com/group/buildcheapeeg/files/Virtual%20Ground/VirtualGnd-TunedCct.pdf
http://groups.yahoo.com/group/buildcheapeeg/files/Virtual%20Ground/VirtualGnd-Tuned.asc
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This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:40 BST