I was reading these threads about seismographs:
http://sydney.edu.au/science/uniserve_science/school/Seismograph/detector/index.html#sept
http://arduino.cc/forum/index.php/topic,56217.0.html.
The stages are: sensor (pendulum, gyroscope, etc) => preamp => analog to digital converter => computer
Why use a preamp as opposed to a 24 bit analog to digital converter like the LTC2400?
If your sensor produces a signal of a few microvolts and your D/A has a reference voltage of 1V then you will be using only a few millionths of the D/A dynamic range.
2^24 = 16,777,216 Microvolts, I used to work on receivers with .1 - .15 uV/12db Sinad and those voltages are 10X 1 bit in 2^24 bits/1V (impedances adjusted) Lower and If I factor in a 6 db drop due to power/voltage relationships that's still close to the noise figure of most fair to good A/D's.
Preamps, SURE if for only improving the signal to noise ratio.
Bob