Hi Chris,

Yes I should have qualified that with "verticals with an output range 
of +/- 10V or +/- 20V ..... can easily vary by a volt or more."  The 
electronics is not the issue.  Any 3-axis or vertical instrument 
supports its mass(es) by some sort of spring, which even in the STS-1 
varies in strength with temperature by some ten or so ppm per deg 
C.  Compared with the accelerations from long-period ground motions, 
1g is a very large acceleration indeed.  It is the variation of 
spring strength with temperature that creates an apparent 
acceleration, causing the 24-hour-period drift.  Only the people who 
are trying to observe earth tides will be concerned with this, and 
for them it is an issue which must be dealt with.  Everyone else just 
filters it out.

Regards,
Brett



>****Brett Nordgren wrote ;
>Even professional instruments show substantial DC drift. Broadband
>verticals have an output proportional to the rate of temperature
>change and over a 24-hour period can easily vary by a volt or more
>depending on their thermal insulation. We routinely put in a
>one-pole digital high-pass filter at the lowest frequency available
>(0.002 Hz in WinSDR) which makes all that go away.
>
>
>****An offset of over a volt could exceed the input range of some ADCs !!
>
>Such poor quality electronics would be totally unacceptable in my opinion.



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