From: Sar Saloth (sarsaloth_at_yahoo.com)
Date: 2002-03-09 03:54:06
Joerg,
Wow, switcherCad is NICE!!!
Using it, I get your results, and with the Tantalum capacitor, it shows my
circuit as unstable. Interestingly, I was able to get components that
worked and was stable over a wide range of load capacitances, from almost
nothing to quite high. however, adding the tantalum with the ESR made it
unstable, no matter what.
At 12:21 AM 2002-03-09 +0100, you wrote:
>Hi,
>
>----- Original Message -----
>From: sleeper75se <sleeper75se_at_yahoo.se>
>To: <buildcheapeeg_at_yahoogroups.com>
>Sent: Friday, March 08, 2002 8:34 PM
>Subject: [buildcheapeeg] Re: Virtual Ground Questions
>
>
> > 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, "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,
>
>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.
>
>During tests of a software (SDL) port of Jim Meissner's synchronizer
>I have looked at my soundcard output:
>It has much ground noise (PC and scope both connected to protection
>earth give a horrible ground loop ) and it has a nasty hp characteristic.
>
>Perhaps I will do the testing with the 555 calibration signal output
>(scaled to 1Vp-p or so)
>
> > > 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
>
>Perhaps a lack of information ?
>
> > if you can fix the problem just by adding a large enough
> > capacitor with low ESR?
>
>Not low ESR but defined ESR. If the ESR gets too low the loop
>gets unstable again.
>
>A very good explanation of how the ESR improves the stability
>can be found in "AN-1148.pdf" from National Semiconductor,
>page 5 chapter "LDO Compensation using 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?
>
>If it is another application than stabilizing voltages (like rail splitters
>and LDOs) a large tantalum C at the output of the amplifier will give you
>a lowpass. It depends on your signal spectrum if this is acceptable or not.
>
>
>Regards,
>
>
>Joerg
>
>
>
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