From: sleeper75se (sleeper75se_at_yahoo.se)
Date: 2002-03-08 19:34:11
Hi Joerg and Sar,
I'm responding to your posts in a single message, to defragment the
discussion a bit.
--- In buildcheapeeg_at_yahoogroups.com, Sar Saloth <sarsaloth_at_yahoo.com> wrote:
> I uploaded a suggested change.
>
> It adds 3 passives (the values are just crude guesses,
> it should be tuned by hand). If the parts are not
> needed at all, just putting jumpers in brings it back
> to the original circuit.
Ok, thanks. I'll make room for them. Jim Meissner will do the tuning
(I hope) and he's got loads of experience with that sort of thing. A
simulation (;-) might improve a bit on those figures beforehand.
[...]
>
> Was there a special reason for using the LT1114?
LOL. Flaunting my ignorance... I have a bunch of them lying around,
and they are precision and low-noise parts. No other reason.
Actually, the part type is not important for me as long as it does
its job well, though I think Linear's products are a notch above the
rest. (No I'm not getting paid for saying that. Maybe I should. :-)
--- In buildcheapeeg_at_yahoogroups.com, Sar Saloth <sarsaloth_at_yahoo.com> wrote:
[...]
> To honor your request for not wasting your time,
> I will volunteer to do the following two things
> since I am to spend the next few weeks searching for
> components anyway
> 1. provide a circuit with 3 extra components that
> will work with ANY OpAmp, but can be tuned or by
> using a jumper would be identical to the current
> circuit. The regulation performance at
> mid-frequencies might be compromised, but then again,
> by adding the components it can be played
> with. Regulation is something that I might
> be willing to simulate.
> 2. Search for the least expensive component
> that claims stability in that type of circuit.
Thanks! I appreciate it.
>
> Speaking of simulators, I have been somewhat disappointed.
[...]
Hmm ... why don't you try these ones (for free):
SwitcherCad III from Linear. Free, allows unlimited circuit sizes,
but lacks some features.
or
Simetrix by Newbury Technologies http://www.newburytech.co.uk Free
version limits circuit size, but it is very good.
> Do those parts tolerate large capacitive coupling?
> How does that impact the total noise figures. I have no
> need for those parts, so I won't investigate them.
> Since there seem to be OpAmps capable of capacitive
> loading, I don't see the use for these parts.
Yes, they are designed to be stable with large caps. In fact there is
a range around 1nF to 1uF where they are unstable for higher currents
than 5mA. Smaller and larger capacitances are ok. Maybe this is the
same effect that Joerg gets with his design; a large low-ESR cap
improves stability rather than deteriorate it? I'm not sure how noise
is affected, probably very little since they must have measured it
under normal operating conditions.
--- In buildcheapeeg_at_yahoogroups.com, "Joerg Hansmann" <info_at_jhansmann.de> wrote:
> 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 ?)
I have all the tools: a function generator, 2 channel 100MHz
oscilloscope, minimum scale 2mV/div, and lab power. What are you
lacking, the function generator, a better scope? Time? :-)
If it is the function generator - how about using the output of a
sound card as signal source? All you need is a pair of biasing
resistors and possibly a capacitor at the opamp input. The resistors
should form a good enough approximation of vref/2 since you are
testing stability and not noise levels.
> 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)
Hmm, then what is all the fuss about in textbooks and application
notes if you can fix the problem just by adding a large enough
capacitor with low ESR? Could it be that most applications that may
be "exposed" to large capacitances, need to output varying voltage
levels rather than DC, so a large cap would simply distort the signal
too much?
Regards,
Andreas
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