On Tue, 2012-10-09 at 08:27 -0400, Brett Nordgren wrote:
> Hi Bob,
> 
> I think I called it 'G' just to correspond with what's used on the 
> geophone's data sheet for sensitivity.  It is indeed a VCVS.

Makes sense - sorry for my confusion!

> The noise runs I have done on one of Dave's circuits, show noise 
> levels far below what I would expect.  I am assuming the noise 
> parameters for the AD706.  I think there's an error somewhere.
> 
> I uploaded a noise plot.  The noise is referred to the model input, 
> that is, equivalent ground noise.  Units are m/sec/sqrt-Hz
> http://bnordgren.org/seismo/NoisePlot.jpg

That does seem low for the AD706.  I don't know the details of your
circuit so hard to compare with the one I posted to this list earlier.
The results for that model (Using LT1007As) is shown here:

  http://c-c-i.com/2.95171615931122/plot0006.ac.png

This is referred to the output and is also in m/sec/sqrt(Hz).  At the
output at one Hz it shows -100dB.  At one Hz my circuit has 45dB of gain
so the equivalent input noise density would be -145dB or about 6E-8.
Not that far off from your results at one Hz.  The LT1007A has input
noise of 60nV (.1 - 10 Hz) versus 500 nV (.1 - 10 Hz) for the AD706.

The integrated noise for my circuit is here:

  http://c-c-i.com/2.95171615931122/plot0007.ac.png

This plot shows about 28 uV at one Hz.  Referring the 28 uV to the input
makes it about 1.6E-7 m / sec equivalent ground velocity.  I don't know
how this compares to "real" seismometers.

The noise table results:

  http://c-c-i.com/2.95171615931122/ntbl0000.ac.txt

show that at one Hz the two op-amps dominate the noise (no surprise!)
and contribute about equally (20 and 18 uV).

-- 
---------------------------------------------------------------------
Bob Smither, Ph.D.                                    bob@...........

"And what country can preserve its liberties, if it's rulers are not
warned from time to time, that this people preserve the spirit of
resistance? Let them take arms."
  -- Thomas Jefferson

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