John,

Another factor that you may want to consider is thermal variation.  At very 
low frequencies, below 1Hz, the effects of micro-variations in the device 
temperature can add additional "noise".  I would imagine that in 
characterizing low-frequency noise, the op-amp manufacturers take pains to 
hold the temperature very steady, which is not what you're going to see in 
the real world.  In real world conditions, it is possible that a device 
with very good temperature properties will perform better (less low 
frequency "noise") than one with much better noise specs, but having larger 
temperature coefficients.

Just an aside, but related, in building very long time-constant RC filters, 
say 1000sec or so, if you use the lowest leakage capacitors, the largest 
noise source seems to be due to the variation of capacitor value with 
temperature and is proportional to the DC voltage on the capacitor.  The 
capacitor charge Q=CV, and in the steady-state can be assumed to be 
constant.  If C goes down due to a temperature change, V goes up--and the 
effect is fairly large.

At very low frequencies, temperature effects look a lot like noise.

Brett


At 08:57 PM 3/9/2005 -0500, you wrote:
>I am getting pretty deeply into the design process for a velocity coil
>seismometer amplifier and filter and have decided to try ot quantify
>the relative noise performance of various candidate opamps versus coil
>resistance.  Here is the list of low noise and general purpose types
>(which I have on hand) that I am comparing:

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