On 12/23/2012 10:10 PM=2C Dale Hardy wrote:
> RE: Instrumentation Question
>
> Hi Geoff=2C
>
> Just an observation=2C
>
> fromthe OPG177datasheet=2C"/The ultralow VOS of the OP177 combines with=20
> its exceptional offset voltage drift (TCVOS) of 0.3 =ECV/=B0C maximum/"
>
> How stable is theOp-ampstemperatureenvironment ?
>
> As you mention=2C capacitor coupling may be the best solution.I have=20
> used 200uF NP capacitorswith OP27's to block tinyDC variations.
>
> Regards=2C
>
> Dale
>
> -----Original Message-----
> From: psnlist-request@.................
> [mailto:psnlist-request@............... On Behalf Of Geoff
> Sent: Monday=2C 24 December 2012 7:16 AM
> To: psnlist@..............
> 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=20
> AC noise level or DC drift of resistance or thermocouple voltage=20
> 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=20
> 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=20
> think) impregnated and has a heavy copper jacket.
>
> It is soldered at the geophone and also at the sensor has soldered=20
> connections.
>
> Reply to this only if you have dealt with such an issue yourself.
>
> I do not want speculations since I swear I have tried every=20
> Uncompensated DC design I can imagine with not being able to stop this=20
> drift. No matter what the design the drift is the same increasing from=20
> morning to afternoon to the + plus and returning to adjusted baseline=20
> in the evening.
>
> The only remaining scheme is a capacitor input AC design ??
>
> Which i did not want to do.
>
> At the moment I rely upon Winquake to filter out the 24 hour DC drift.
>
> __________________________________________________________
>
> Public Seismic Network Mailing List (PSNLIST)
>
> To leave this list=20
> emailPSNLIST-REQUEST@..............with=20
> the body of the message (first line only): unsubscribe=20
> Seehttp://www.seismicnet.com/maillist.htmlfor more information.
>
The temps are as stable as my air conditioner
several degrees F of drift. But the drift of the amp relates to outside
and not inside conditions. Only the geophone and cable are located=20
outside=2C underground.

That was my final solution. It seems possible to create an AC amplifier=20
with a DC gain of x1.
It seems to work quite well. Also=2C if i break up the input resistors and=
=20
filter
with small capacitors I get rid of the higher frequency AC junk (RFI).
Im using 4700uf caps because if in for a penny also in for a pound.
But only my preamp is using such a design.
I might try doing with both pre and post amps but those 4700Uf take up=20
too much space.

regards=2C
geoff



  
    
  
  
    
On 12/23/2012 10:10 PM=2C Dale Hardy
      wrote:

    
    

      
      
      
      
      
Hi
            Geoff=2C
      

      
Just
            an observation=2C 
      
from the OPG177
            datasheet=2C " Th=
e
              ultralow VOS of the OP177 combines with its exceptional
              offset voltage drift (TCVOS) of 0.3 =ECV/=B0C maximum<=
/i> "

      
How stable is the Op-amps<=
/span> temperature environment ?

      
As
            you mention=2C capacitor coupling may be the best solution. I have used
            200uF NP capacitors with OP27's to block tiny DC variations.<=
span
          lang=3D"en-au">

      
Regards=2C
      
Dale
      

      
-----Original Message-----

            From: psnlist-request@.............. [mailto:psnlist=
-request@..............]
            On Behalf Of Geoff

            Sent: Monday=2C 24 December 2012 7:16 AM

            To: psnlist@..............

            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.
      
Reply
            to this only if you have dealt with such an issue yourself.

      
I
            do not want speculations since I swear I have tried every
            Uncompensated DC design I can imagine with not being able to
            stop this drift. No matter what the design the drift is the
            same increasing from morning to afternoon to the + plus and
            returning to adjusted baseline in the evening.
      
The
            only remaining scheme is a capacitor input AC design ??<=
/span>
      
Which
            i did not want to do.
      
At
            the moment I rely upon Winquake to filter out the 24 hour DC
            drift.
      
__________________________________________________________
      
Public
            Seismic Network Mailing List (PSNLIST)
      
To
            leave this list email PSNLIST-REQUES=
T@.............. with the body of the message
            (first line only): unsubscribe See http://www.sei=
smicnet.com/maillist.html for more information.
    

    The temps are as stable as my air conditioner

    several degrees F of drift. But the drift of the amp relates to
    outside

    and not inside conditions. Only the geophone and cable are located
    outside=2C underground.

    

    That was my final solution. It seems possible to create an AC
    amplifier with a DC gain of x1.

    It seems to work quite well. Also=2C if i break up the input resistors
    and filter

    with small capacitors I get rid of the higher frequency AC junk
    (RFI).

    Im using 4700uf caps because if in for a penny also in for a pound.

    But only my preamp is using such a design.

    I might try doing with both pre and post amps but those 4700Uf take
    up too much space.

    

    regards=2C

    geoff