> ChrisAtUpw@....... wrote:

>     You might take a look at:-
> 
>     Op Amp Selection Guide for Optimum Noise Performance
> http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1021,P2440,D6539

I have this, but am trying to be able to reproduce it from the data
sheets (so far, I think my results do not contradict this, so I think
I am doing it right), and referenced to the much lower frequency range
we need, as well as include other brands.  Something interesting to me
that does not show up in the list I posted, but in the graphs is the
fact that the overall signal to noise ratio climbs as the coil wire
size is reduced, even though it results in more resistive noise.  But
some opamps have such low current noise and the extra turns provide
more signal voltage, so that there is a steady climb in signal to
noise ratio to coil resistances around 100k, and then there is a
second, even bigger peak for coil resistances of 100 meg ohms, but I
wouldn't want to handle the wire.  ;-)

But the graph does show that there is signal to noise value in going
with the smallest size wire you can deal with.
 
>      See also www.linear.com AN87 pp87-89 BRIDGE MEASURES SMALL
> CAPACITANCE
> IN PRESENCE OF LARGE STRAYS by Jeff Witt. It should work fine with
> seismometer capacitor plates, but will need a low pass output filter
> added.

Will do.
Thanks.

-- 
John Popelish
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