PSN-L Email List Message

Subject: Re: Velocity Sensor Question ?
From: Brett Nordgren brett3nt@.............
Date: Mon, 19 Jul 2010 17:46:51 -0400


Hi Goeff,

At 07:16 PM 7/18/2010, you wrote:
>Here is the preamp.
>I was going to place it right at the geophone.
>The two exact coils will change in step
>with temperature to keep a stable DC
>baseline. Minimal drift.
>
>http://gmvoeth.home.mchsi.com/AMP001.jpg
>
>Any reason this should not be right ?

It's almost right.  I think you intended the "-" input to be the top 
terminal, so that the resistor from the output goes to "-" and 
doesn't make positive feedback.  Another small correction is that for 
perfect balance that particular resistor needs to be 27k - 250 = 
26,750.   Or, easier, you could just connect the coils in series and 
hook the pair to a single-ended op-amp circuit.  That way the 
magnetics could effectively be differential while still using a 
simpler single-ended op-amp connection.

The coil resistance variations don't cause DC baseline shifts (see 
below), but they do cause the instrument sensitivity to vary by about 
0.4% per deg/C, which will still be the case, even with a 
differential circuit.  I believe that the very best (k$$) commercial 
instruments vary in sensitivity by more than 0.03% per degree, so for 
a home made instrument 0.4%/degC doesn't seem to be all that terrible.

And If you felt you needed to, you could reduce the effect of the 
coil resistance variation on the gain by a factor of 11 by adding a 
2.5K resistor in series with each of the coils and then increasing 
the 27K resistors to something in the vicinity of 270 K to restore 
the gain to what you had before.

>Will Larry sell only the coils ?

You might try e-mailing him.

>I might get two to try my ideas.
>
>I looked into getting ten wound but the company
>was outragious in its (retooling fees)
>It seems they cant simply do it even tho they
>wind the things for a living.

When we were making transformers we found that setting up make a 
particular part that's not in inventory can take considerable time. 
(=$$)  Once set up you can turn them out very fast, but then you have 
to make hundreds at a time to be able to sell them for a reasonable 
price and still cover the setup costs.

**********
>I understand the stability of the baseline is related to the
>DC nature of the balance between the + and - legs
>of the operational amplifier. Copper has
>a temperature coefficient which means it changes at a
>certian rate its resistance with temperature.
>If i balance both sides of the equation with identical
>physical properties, then I should expect
>they will change identically with temperature
>thus keeping the DC baseline steady.
>If you use only one copper coil on one leg this
>balance becomes impossible to achieve.
>This device will be located outside where
>temp changes are greatest.

What we have found to be most important are output voltages, "DC 
baseline shifts", which appear when there is no ground motion (we'll 
call them instrument noise voltages).  With no ground motion, the 
coil voltages are zero and an ideal amplifier will have zero output 
regardless of the coil or other resistance values.  The most likely 
source of noise signals will be from inside the op amp.

>I do not know where you guys are coming from
>with your stable DC baselines unless you use
>some kind of miracle device which I have never found.

Nope, just good op-amps in good circuits.  Bear in mind that even a 
single-ended op-amp circuit is still differential in a number of 
important ways.

Regards,
Brett



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