Surrounding the amp and filter circuits with some type of material to
reduce air flow is a very good idea - especially for very high gain
circuits.  I enclosed my entire setup with 2 inch thick panels of foam
insulation.  For a while I kept the chamber at a constant 70 degrees (the
seismograph is located under my house which stays relatively temp stable
already) but decided that the cost of eclectricity to maintain the temp
was not worth it.

Art




> I've seen this effect with thermocouple amplifiers, where moving your hand
> near the circuit would move the air enough to create low-frequency noise.
> It can be almost eliminated by pressing the circuit board between pieces
> of foam rubber.
>
> Jack
>
> -----Original Message-----
> From: psn-l-request@.............. [mailto:psn-l-request@...............
> On
> Behalf Of Brett Nordgren
> Sent: Thursday, March 10, 2005 7:52 AM
> To: psn-l@..............
> Subject: Re: Op amp front end noise
>
> 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:
>
>                   If my e-mail address above is not working
> you can always reach my mail form at: http://bnordgren.org/contactB.html
>                             using your Web browser.
>
>
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