macsmith wrote:
> 
> I'm about to put a simple 10x or 100x pre-amp onto my 4.5 Hz geophone
> which I recently got from the Group-Buy.
> 
> What sort of damping circuit should I put in front of the amplifier to
> prevent the geophone from simply oscillating?  Also, what attenuation
> would this represent?
> 
> Sorry if this has already been covered.
> 
> Cheers
> 
> Ian Smith
> 

If you don't damp it at all, it won't oscillate because
there is some intrinsic damping.  Worst case you will get
about twice the output at the natural frequency.

If you design a classic 2-resistor inverting op amp circuit,
then the input resistor becomes the damping resistor, so
don't make it too small.  I looked at a couple of data
sheets and for one geophone the right answer was about 500
ohms, for another maybe 2K.  I would start with 1K.

If you design a non-inverting amplifier, then you can put a
damping resistor in parallel with the geophone.  If you plan
on some distance between the geophone and the amplifier, you
should design a floating differential amplifier to avoid hum
and noise.

If somebody will refresh my memory on the model number and
coil resistance, I might have a data sheet.

Doug Crice
-- 
Doug Crice          web site http://www.georadar.com
GeoRadar Inc.             e-mail dcrice@............
19623 Via Escuela Drive           phone 408-867-3792
Saratoga, CA 95070 USA              fax 408-867-4900

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