On 12/24/2012 12:49 PM, RSparks wrote:
> Hi Geoff,
>
> By "DC drift", do you mean that your traces are not centered on the 
> zero Y line?  If not, how do  you observe that you have a "small DC 
> drift?
>
> Roger
>
> On 12-24-2012 12:01 AM, psnlist-digest-request@.............. wrote:
>> .------ ------ ------ ------ ------ ------ ------ ------ ------ ------.
>> | Message 1                                                           |
>> '------ ------ ------ ------ ------ ------ ------ ------ ------ ------'
>> Subject: Instrumentation Question
>> From:    Geoff 
>> Date:    Sun, 23 Dec 2012 20:15:55 +0000
>>
>> 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 ??
>> Which i did not want to do.
>>
>> At the moment I rely upon Winquake to filter out the 24 hour DC drift.
>>
>>
>>
>
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>

I will center the DC baseline using pins 1 and 8.
Over the course of a day this baseline drifts up during the day
and back down at night.

I have found one way to deal with this.

You must create an AC amplifier with a DC gain of
X1 and an AC gain of what you want.

Then you zero each stage and they will stay on Zero
except for the desired signals.

regards,
geoff
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