Gary,

You may want to consider 0.5kg as a maximum.  Even slightly lower might be 
easier to work with in a feedback design.  Also, positioning the forcing 
coil at a greater distance from the pivot than the mass, has the effect of 
reducing the effective mass by the ratio of their distances.

It is likely that a 16-bit D/A will be the principal limit.  From what I 
have been told, if you make its bit sensitivity high enough to see 
microseisms, the digital clipping level will end up being fairly 
low.  Others who are doing that may want to comment on what they are 
getting.  It's only when you go to a 24 bit digitizer that you can have 
high sensitivity to weak signals and still be able to display large 
mid-distance quakes without clipping.  The instrument itself will probably 
not be the limit when using 16 bits.  The highest coil currents will be 
needed at the highest frequencies, so if you don't push for too high an 
upper corner frequency, you should be also a little better off.

To understand the clipping issues for your location, determine what your 
maximum acceleration is likely to be, often expressed in % of g, then use F 
= m A to determine the peak force and calculate what that corresponds to in 
terms of coil current.  I suspect that's what you have already 
done.  Balancing sensitivity vs clipping level seems to be a fundamental 
problem, which often results in the use of more than one type of instrument 
in seismicly active areas.

Regards,
Brett

At 07:46 PM 9/12/2008 -0700, you wrote:
>Brett,
>Thank you for the answer to my question. I dusted off my balance instrument
>and I think I have a DC supply, so I'm all set to measure newtons/amp. I can
>see that the average current is going to be quite low, but if M6 or M7
>earthquake a 1000 miles away is observed, I would think the peak current
>could be high, provided all things are in the linear range. I'm assuming the
>pendulum mass is about .5 Kg.
>Gary


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