Barry,

You asked about the 600 ohm transformer between the excitation oscillator 
and the bridge in the displacement transducer:

The reason that an isolation transformer is used in all reactive bridge
circuits (VRDT, LVDT, and capacitive) is to remove any DC component
from the excitation signal. Even the most perfect amplifier  at the
output of the oscillator circuit will have some small DC offset voltages 
present at the output, and these will be modulated by all the noises 
present at its input and amplified to the output. 

Without a transformer, these DC noise currents flowing in the inductors 
of the VRDT or LVDT will cause magnetic forces on the moving transducer 
sensing element, causing noise forces on the seismometer mass. Magnetic 
forces are not a problem for capacitive sensors, but electrostatic forces 
are, especially for large plate areas and sub-millimeter spacing. And even 
sub-millivolt DC noise can have large electrostatic effects.

So there is nothing more simple than the transformer to isolate the DC
noise. And some bridge circuits actually use a transformer in the output
to provide a 10x to 100x voltage gain prior to the bridge AC amplifier,
which also prevents input offset currents of the AC amplifier from getting
back into the transducer. Often large capacitors are also used between the
bridge and the amplifier to block any DC noise. This is particularly
important for electrolytic tiltmeter bubbles, where any DC current will
plate the electrodes off the vial walls. So it is also necessary to use
an op-amp at the bridge output amplifier with minimal input current noise
and DC offset.

The 600/600 ohm transformers are available from Mouser, Newark, and
others for a few dollars.

Regards,
Sean-Thomas

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