In a message dated 14/03/2005 16:53:43 GMT Standard Time,  jpopelish@........ 
writes:

Something interesting to me that does not show up in the list I posted,  but 
in the graphs is the fact that the overall signal to noise ratio climbs as  
the coil wire size is reduced, even though it results in more resistive  noise.  
But some opamps have such low current noise and the extra turns  provide more 
signal voltage, so that there is a steady climb in signal to  noise ratio to 
coil resistances around 100k, and then there is a second, even  bigger peak 
for coil resistances of 100 meg ohms, but I
wouldn't want to  handle the wire. 
Hi John,
 
    The opamps have a design impedance when the current  and voltage noise 
levels are about equal. If the coil resistance is less than  this, it pays to 
add turns. 
 
    Are you taking the 1/f noise into account? This is  usually fairly 
critical for seismic sensors, particularly when you are  considering long period 
types.

But the  graph does show that there is signal to noise value in going
with the  smallest size wire you can deal with.
    It will usually pay to choose a fairly low  amplifier impedance for 
inductive systems. The coils are much easier to make and  physically smaller, which 
allows you to take full advantage of the very high  fields that can be 
produced by 'modern' NdFeB magnet systems.
 
    The larger the coil, the more difficult it is to  screen it from 
environmental noise. In general, most of us do not have the  luxury of quiet seismic 
sites. The larger the inductance, the more susceptible  is the wiring to 
picking up stray signals. It can pay to put a ceramic capacitor  across the input to 
the opamp. The use of screened cable with a large dielectric  loss can be an 
advantage. 
 
    Regards,
 
    Chris Chapman







In a message dated 14/03/2005 16:53:43 GMT Standard Time,=20
jpopelish@........ writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000=20
  size=3D2>Something interesting to me that does not show up in the list I p=
osted,=20
  but in the graphs is the fact that the overall signal to noise ratio climb=
s as=20
  the coil wire size is reduced, even though it results in more resistive=20
  noise.  But some opamps have such low current noise and the extra tur=
ns=20
  provide more signal voltage, so that there is a steady climb in signal to=20
  noise ratio to coil resistances around 100k, and then there is a second, e=
ven=20
  bigger peak for coil resistances of 100 meg ohms, but I
wouldn't want t=
o=20
  handle the wire. 
Hi John,
 
    The opamps have a design impedance when the cur=
rent=20
and voltage noise levels are about equal. If the coil resistance is less tha=
n=20
this, it pays to add turns. 
 
    Are you taking the 1/f noise into account? This=
 is=20
usually fairly critical for seismic sensors, particularly when you are=20
considering long period types.
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>But the=20
  graph does show that there is signal to noise value in going
with the=20
  smallest size wire you can deal with.
    It will usually pay to choose a fairly low=20
amplifier impedance for inductive systems. The coils are much easier to make=
 and=20
physically smaller, which allows you to take full advantage of the very high=
=20
fields that can be produced by 'modern' NdFeB magnet systems.
 
    The larger the coil, the more difficult it=20=
is to=20
screen it from environmental noise. In general, most of us do not have the=20
luxury of quiet seismic sites. The larger the inductance, the more susceptib=
le=20
is the wiring to picking up stray signals. It can pay to put a ceramic capac=
itor=20
across the input to the opamp. The use of screened cable with a large dielec=
tric=20
loss can be an advantage. 
 
    Regards,
 
    Chris Chapman