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Subject: Re: Assistance needed understanding this please !
From: "Geoffrey" gmvoeth@...........
Date: Tue, 21 Nov 2006 21:18:03 -0700

Hello Chris;

I might seem to ramble here because
I am not in possession of academic discipline.

I have been playing around for the past 10 years
(maybe several months of real time) and have
learned that the only decent preamps for a geophone
seem to be ones with PHANTOM grounding in the input coil.
this means no matter what the coil resistance is
there exists a mid point inside the coil you might call
a ground. You just need to use a totally shielded
line to the geophone tied to circuit ground
to keep out unwanted electrical stuff.

The gain will drift a bit with temp but not the baseline.
you seem to need not more than X10 in the preamp
to keep the gain changes small.
Not less than X10 because of effects on CMMR ??

So long as you tie the geophone directly across
the +/- inputs in a differential fashion you seem
to get a response similar to a full blown
instrumentation amplifier.

It is the only way to avoid the resistance vs temp
changes from affecting the class "A" operational

The circuit you see is just one of many
I play around with through curiosity.

The original ideas most all come from the
PDF files that you can download relating to
whatever part the company has sold you.
They are full of various circuits you can
experiment with.

The differential amp always shows two devices
with a ground in the middle and I have found this
is not necessary. Just hook a coil between
+ and - of an op amp and the gain is (Rf/(0.5 Rgeo))
Very similar to an instrumentation arrangement
of three opamps.

The internal resistance between + and - is 40 megs or so.
Depends upon the op amp in question.
I use OP177G from the Philippines.
They can not be loaded greater than about 2K on the output.
( greater load means smaller R)

The OP177F are very rugged and it takes an act of congress
to destroy them but they are not made anymore I think.
They have a ceramic DIP case instead of cheap plastic.

What I am not certain of is there a natural
DC current flow between + and - or do they always
float at the same potential ??

The circuit I showed you seemed to work well
but because it puts a common mode signal
on the input I am reluctant to use it.

Instead I am simply now using a gain of X10
on the preamp and tying the + side to the ground
using a resistor matched with Rf (R feedback)
within the limits of my
test equipment.

I have found you need to match the + and - sides of
these within maybe 0.1% or your common mode rejection ratio
is not so good.

I could not make the negative resistance circuit work
for me that is in the paper:
'Overdamping geophones using negative impedances'
Bernd Ulmann
ulmann@......... (may no longer be valid)

It really seems to work opposite of what it says
by increasing not decreasing the eigen frequency.
and at the same time lowering the sensitivity
of the sensor device.

instead of locking the mass
I think you need to somehow artificially decrease the
spring constant or simulate electronically
a greater mass to get the desired results.

by whatever means I need to create a
vertical sensor with a free period of
three to four seconds.

I have been considering a push-pull arrangement
using unity gain amps to force/drive the opamp to behave
differently than normally possibly to lower
the eigen freq abnormally to 4 seconds.

It is just a thought and have not seriously looked into
this yet.

Can you please tell me how the given circuit affectes
the geophone
because I never got above college algebra and that was
20 years ago.

I am going to scan all the drawings of my current setup
into a zip file and let anyone interested have a copy.
You can all do whatever you want with my ideas
I have no interest in making money or anything
just having fun with the science of Geology
and Electronics and programming.
I just need help from those who know how to
properly (electronically) lower the free period
of a mass/spring system to about 4 seconds so
I can use a straight forward amplification
and have it be linear in gain between
3 seconds and 3 hertz.

I think this might really be done somehow
with a narrow band high Q feedback circuit
into a push pull arrangement of X1 opamps.
To artificially lower the spring constant.
Never tried this and am afraid because I do not want to
accidentally damage my geophone magnet or coil.

To anyone else reading this links on my home page
do not point to things forever. In a month or so
I will remove the files meaning they will no longer
be there if you try to look see.
If you are interested you must save these
files to your HD or contact me later
and ask to see them again.


----- Original Message ----- 
Sent: Sunday, November 19, 2006 9:04 AM
Subject: Re: Assistance needed understanding this please !

> In a message dated 2006/11/19, gmvoeth@........... writes:
>> Does anyone understand how this circuit will affect the geophone ?
>> If so can you show me in laymans terms what is happening ?
> Hi Geoff,
>       I haven't worked out the balance equation, but this is a -ve input 
> impedance circuit, presumably to very heavily damp the geophone. This should / 
> could cause the output to have a 1/f characteristic, rather than the normal 
> dogleg, flat down to the resonance and 1/f^2 below this. 
>       This type of circuit is used to 'broad band' a geophone response. 
> Where did it come from?
>       Regards,
>       Chris Chapman

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