DC drift is very common --almost universal -- in seismometers .   Even the 
best broadband instruments have a drift proportional  to the rate of 
temperature change.   The frequency content of the drift  are usually well 
below the seismic information so is of no consequence -- just filter it out 
with a high pass at a frequency below the lowest frequency of significance 
for your  instrument.

Without a good cover the best broadbands can become quite noisy at long 
periods and appear to be drifting.  An STS-2 must be very well insolated 
from temperature change to reduce  instrument self noise  at the long 
periods  in order to observe the earth normal  modes with a good signal to 
noise ratio.

A 24 hour periodic drift is of absolutely no consequence for a geophone. For 
a 1 second instrument ,even with period extension, the longest practical 
seismic signal which can be observed is probably around 20 seconds . The 
noise level at 20 seconds, after period extension, will be so high that only 
very large events will be observable above the instrument/system noise.  A 
high pass  filter in Winsdr at .05 Hz 1 pole or AC coupling with a time 
constant of about 3 seconds ( 20 seconds / 2pi)  would be about right.

It sounds like you are doing just fine, just filter and forget.

Cheers,
Dave Nelson
Rolling Hills Estates, California



--------------------------------------------------
From: "Geoff" 
Sent: Monday, December 24, 2012 1:40 PM
To: 
Subject: Re: Instrumentation Question

> On 2012-12-24 1:29, chrisatupw@....... wrote:
>> From: Geoff 
>> To: psnlist 
>> Sent: Sun, 23 Dec 2012 20:17
>> Subject: Instrumentation Question
>>
>> I have tried every op amp circuit I can imagine as a preamp.
>>
>> Yet I can not rid a small DC drift which is either related to
>> ambient AC noise level or DC drift of resistance or
>> thermocouple voltage related to the soldering of junctions.
>>
>> This DC drift is related to the 24 hour cycle.
>>
>> The dc drift is on the order of microvolts
>> which seem to be originating on the input.
>>
>> The overall DC gain is 80Dbv or X10000.
>>
>> I should be able to achive a DC free drift at this gain.
>>
>> I am using a op177G op amp ??
>>
>> The sensor is HS10-1 Geophone.
>>
>> The 40 foot of cable is designed for burial
>> it has silicon grease (I think) impregnated and
>> has a heavy copper jacket.
>>
>> It is soldered at the geophone and also at the sensor
>> has soldered connections.
>>
>> Hi Goeff,
>>
>>
>>
>>      If you use a CAZ type opamp like the LTC1150 you will get zero 
>> temperature drift.
>>
>> Use a DIL socket soldered to your board, grease the pins of the opamp 
>> with Vaseline
>>
>> and simply plug it in.Soldering it in can makean opamp  noisy, if youheat 
>> it too much.
>>
>>      Why are you using so much gain ? Geophones normally require much 
>> less.
>>
>>      Look up the OP177 spec ! Which one are you using  ?The G version 
>> drifts 1.2 micro
>>
>> V / C Deg !  !
>>
>>      Regards,
>>
>>       Chris Chapman
>>
>>
>>
>
> The problem does not seem to be the amplifier itself.
> But rather, the variables dealing with the input.
> Cable/Geophone/Common mode stuff/
> Yet I have found a wide variation within the
> op amp offset voltages.
> I am currently using a Chopper amp (or so I think)
> In the front end. LTC1050 ???
> Yes, I have troubles finding decent opamps
> at civilian prices, the best are all seem to be MilSpec
> rip offs. Outrageous prices. The milspec parts are by far
> the best and all others should simply be scrapped.
> Or so i believe. if all you create are the best,
> then the best will be the cheapest it can be.
> The trouble here is, administrators treat human beings
> like technicians treat inanimate things.
> ripping off their bad opamp legs so they cant be used elsewhere
> in other circuits in the future.
> Regards,
> geoff
> LTC1050 - Precision Zero-Drift Operational Amplifier with Internal 
> Capacitors
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