Hi Goeff=0Ayou might also be able to put=A0some zeroing circuitry using a t=
rim pot in the =0Ainput end to adjust the zero. I'm not sure how that would=
 be configured for your =0Acircuit. =0A=0A=A0Regards=0ABarry=0Ahttp://www.s=
eismicvault.com =0A=0A=0A=0A=0A________________________________=0AFrom: Bre=
tt Nordgren =0ATo: psn-l@...................... Mon=
, July 19, 2010 2:46:51 PM=0ASubject: Re: Velocity Sensor Question ?=0A=0AH=
i Goeff,=0A=0AAt 07:16 PM 7/18/2010, you wrote:=0A>Here is the preamp.=0A>I=
 was going to place it right at the geophone.=0A>The two exact coils will c=
hange in step=0A>with temperature to keep a stable DC=0A>baseline. Minimal =
drift.=0A>=0A>http://gmvoeth.home.mchsi.com/AMP001.jpg=0A>=0A>Any reason th=
is should not be right ?=0A=0AIt's almost right.=A0 I think you intended th=
e "-" input to be the top =0Aterminal, so that the resistor from the output=
 goes to "-" and =0Adoesn't make positive feedback.=A0 Another small correc=
tion is that for =0Aperfect balance that particular resistor needs to be 27=
k - 250 =3D =0A26,750.=A0 Or, easier, you could just connect the coils in s=
eries and =0Ahook the pair to a single-ended op-amp circuit.=A0 That way th=
e =0Amagnetics could effectively be differential while still using a =0Asim=
pler single-ended op-amp connection.=0A=0AThe coil resistance variations do=
n't cause DC baseline shifts (see =0Abelow), but they do cause the instrume=
nt sensitivity to vary by about =0A0.4% per deg/C, which will still be the =
case, even with a =0Adifferential circuit.=A0 I believe that the very best =
(k$$) commercial =0Ainstruments vary in sensitivity by more than 0.03% per =
degree, so for =0Aa home made instrument 0.4%/degC doesn't seem to be all t=
hat terrible.=0A=0AAnd If you felt you needed to, you could reduce the effe=
ct of the =0Acoil resistance variation on the gain by a factor of 11 by add=
ing a =0A2.5K resistor in series with each of the coils and then increasing=
 =0Athe 27K resistors to something in the vicinity of 270 K to restore =0At=
he gain to what you had before.=0A=0A>Will Larry sell only the coils ?=0A=
=0AYou might try e-mailing him.=0A=0A>I might get two to try my ideas.=0A>=
=0A>I looked into getting ten wound but the company=0A>was outragious in it=
s (retooling fees)=0A>It seems they cant simply do it even tho they=0A>wind=
 the things for a living.=0A=0AWhen we were making transformers we found th=
at setting up make a =0Aparticular part that's not in inventory can take co=
nsiderable time. =0A(=3D$$)=A0 Once set up you can turn them out very fast,=
 but then you have =0Ato make hundreds at a time to be able to sell them fo=
r a reasonable =0Aprice and still cover the setup costs.=0A=0A**********=0A=
>I understand the stability of the baseline is related to the=0A>DC nature =
of the balance between the + and - legs=0A>of the operational amplifier. Co=
pper has=0A>a temperature coefficient which means it changes at a=0A>certia=
n rate its resistance with temperature.=0A>If i balance both sides of the e=
quation with identical=0A>physical properties, then I should expect=0A>they=
 will change identically with temperature=0A>thus keeping the DC baseline s=
teady.=0A>If you use only one copper coil on one leg this=0A>balance become=
s impossible to achieve.=0A>This device will be located outside where=0A>te=
mp changes are greatest.=0A=0AWhat we have found to be most important are o=
utput voltages, "DC =0Abaseline shifts", which appear when there is no grou=
nd motion (we'll =0Acall them instrument noise voltages).=A0 With no ground=
 motion, the =0Acoil voltages are zero and an ideal amplifier will have zer=
o output =0Aregardless of the coil or other resistance values.=A0 The most =
likely =0Asource of noise signals will be from inside the op amp.=0A=0A>I d=
o not know where you guys are coming from=0A>with your stable DC baselines =
unless you use=0A>some kind of miracle device which I have never found.=0A=
=0ANope, just good op-amps in good circuits.=A0 Bear in mind that even a =
=0Asingle-ended op-amp circuit is still differential in a number of =0Aimpo=
rtant ways.=0A=0ARegards,=0ABrett=0A=0A=0A=0A______________________________=
____________________________=0A=0APublic Seismic Network Mailing List (PSN-=
L)=0A=0ATo leave this list email PSN-L-REQUEST@.............. with =0Athe b=
ody of the message (first line only): unsubscribe=0ASee http://www.seismicn=
=0A
Hi Goeff
=0A
you might also be =
able to put some zeroing circuitry using a trim pot in the input end t=
o adjust the zero. I'm not sure how that would be configured for your circu=
it. 
 
Regards
Barry
http://www.seismicvault.com =0A
<=
BR>
=0A

=0A
=0A
=
=0AFrom: Brett Nordgren <=
;brett3nt@.............>
To: psn-l@..............
Sent:<=
/SPAN> Mon, July 19, 2010 2:46:51 PM
Subject: Re: Velocity Sensor Question ?

Hi G=
oeff,

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 c=
oils will change in step
>with temperature to keep a stable DC
>=
;baseline. Minimal drift.
>
>http://gmvoeth.home.mchsi.com/AMP0=
01.jpg
>
>Any reason this should not be right ?

It's alm=
ost right.  I think you intended the "-" input to be the top 
termi=
nal, 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 =3D 
26,750.&nbs=
p; 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 cou=
ld effectively be differential while still using a 
simpler single-ended=
 op-amp connection.

The coil resistance variations don't cause DC ba=
seline 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 wi=
th a 
differential circuit.  I believe that the very best (k$$) com=
mercial 
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 ter=
rible.

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 onl=
y the coils ?

You might try e-mailing him.

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

=
When we were making transformers we found that setting up make a 
partic=
ular part that's not in inventory can take considerable time. 
(=3D$$)&n=
bsp; 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
>ce=
rtian rate its resistance with temperature.
>If i balance both sides =
of the equation with identical
>physical properties, then I should ex=
pect
>they will change identically with temperature
>thus keepi=
ng the DC baseline steady.
>If you use only one copper coil on one le=
g this
>balance becomes impossible to achieve.
>This device wil=
l be located outside where
>temp changes are greatest.

What we=
 have found to be most important are output voltages, "DC 
baseline shif=
ts", which appear when there is no ground motion (we'll 
call them instr=
ument noise voltages).  With no ground motion, the 
coil voltages a=
re 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 
im=
portant ways.

Regards,
Brett



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