In a message dated 04/07/02, furansowa@........... writes:

> could someone explain me why you need 24bits AD converters in seismology.
> Indeed it corresponds to 16.8millions points on a 2g scale, which thus 
> corresponds to a resolution of about 120 nano-g (10^-12g !)
> 

Hi Francois,

       Some, but not all the '24 bit' A/Ds do give 24 bits if you take 
samples slowly enough. The range for 'normal' sampling rates may be 18 to 22 
bits. They will only effectively handle one channel / sensor at a time - the 
settling time on channel change tends to be rather long.
       The idea is that you don't then need amplifiers for your sensors and 
the one range will handle all possible signal levels, so you read a Geophone 
etc directly into the A/D. However, it may be more than a bit difficult to 
sense signals less than 100 nano Volts. I do not know what is done about the 
several micro volts of thermo-electric potentials that you will inevitably 
get, though.
       These and many other technical issues are described in detail in a 5.6 
MB file at http://www.ifjf.uib.no/seismo/SOFTWARE/OTHER/instrument.pdf
       This looks a very good 'book' and well worth keeping as a reference.

       Regards,

       Chris Chapman
In a message dated 04/07/02, furansowa@........... writes:



could 
someone explain me why you need 24bits AD converters in seismology.

Indeed it corresponds to 16.8millions points on a 2g scale, which thus corresponds to a resolution of about 120 nano-g (10^
-12g !)

Isn't the ambient noise much higher than this ?



Hi Francois,



       Some, but not all the '24 bit' A/Ds do give 24 bits if you take samples slowly enough.
 The range for 'normal' sampling rates may be 18 to 22 bits. They will only effectively handle one channel / sensor at a time -
 the settling time on channel change tends to be rather long.

       The idea is that you don't then need amplifiers for your sensors and the one range wil
l handle all possible signal levels, so you read a Geophone etc directly into the A/D. However, it may be more than a bit diffi
cult to sense signals less than 100 nano Volts. I do not know what is done about the several micro volts of thermo-electric pot
entials that you will inevitably get, though.

       These and many other technical issues are described in detail in a 5.6 MB file at http
://www.ifjf.uib.no/seismo/SOFTWARE/OTHER/instrument.pdf

       This looks a very good 'book' and well worth keeping as a reference.



       Regards,



       Chris Chapman