PSN-L Email List Message

Subject: RE: Instrumentation Question
From: "Blair Lade" blairl@...............
Date: Mon, 24 Dec 2012 16:20:39 +1030


You really don't have much choice, there are zero drift IC's that are
switched capacitor sampling types, but and it's a big but, there will be
drift from other components and thermocouple effects will start to come into
play 

AC coupling on the output of the OP amp (not the input) is the best way to
remove slow drifts from temperature effects.

Do you have a lo pass filter before your digitizer?
The input to that would normally be 'hi level' and AC coupled..
Offcourse, there may well, be temperature related drifts associated with the
filter circuit(s) as well.

Drifts with reference voltage sources in ADC circuits are also common, and
good quality resistors, good soldering skills and stable temperatures are
required for precision circuits.

Barometric pressure changes can also cause 'DC types' of low frequency
voltage drifts, particularly in capacitors 

You shouldn't need Dc coupling to the digitizer from your hi gain stage for
seismic signals from earthquakes.

And 
I question the need for so much gain..that will give you some grief if you
are trying to extend the low freq response by severely rolling off the hi
end

Blair (Australia)

-----Original Message-----
From: psnlist-request@.............. [mailto:psnlist-request@...............
On Behalf Of Geoff
Sent: Monday, 24 December 2012 6:46 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 ??
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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