PSN-L Email List Message

Subject: Re: Geophone electrical model update
From: Brett Nordgren brett3nt@.............
Date: Tue, 09 Oct 2012 22:28:57 -0400

Hi Bob,

The AD706 is quite a low-noise device.  Typically 15E-9 V/sqrt-Hz 
with 1/f corner at 4Hz and 50E-15 A/sqrt-Hz with 1/f corner at 8Hz.

At lowest frequencies the LT1007A looks like it typically has 12x 
lower voltage noise and 400x higher current noise than the 706.  I 
need to look at how it would work in some of the higher-impedance 
locations of our circuits.  Also Dave has generally used dual op amps 
to simplify the board and keep its size down.  Though I suspect that 
in seeking the best voltage noise performance one finds that dual 
devices suffer from some degradation due to thermal interaction 
between the two circuits.

At this point the broadband designs generally have lower noise than 
the local background, though at Dave's excellent site that may not be 
entirely true, below about 20 mHz.  What I really like to know is 
which parts are generating how much noise and why.  So far the 
position sensor has resisted noise modeling.  Measurements are in order.


At 09:23 AM 10/9/2012, you wrote:

>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
> >
>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:
>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:
>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:
>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@...........


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