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Subject: AD7745 (was AD7142 ?)
From: Brett Nordgren Brett3kg@.............
Date: Thu, 13 Apr 2006 09:48:15 -0400


To see what the 7745 could do, I assumed a balanced sensor with plates=20
spaced 1mm on either side of the moving plate and found I needed an area of=
3.4 cm^2 to best use the range of the 7745 (+/- 4pF).  Then I assumed a=20
sampling rate of 50Hz.  The biggest issue is that the output is digital and=
that presents some serious problems when you try to design it into a=20
feedback instrument.

At 09:25 PM 4/12/2006 -0700, you wrote:
>Hi Brett
>   STM wrote as a solution to LVDTs "The common solution is to use a=20
> capacitive bridge transducer, where a moving vane moves between two fixed=
> plates that form a capacitive bridge that is unbalanced by the movement=20
> of the vane. With proper geometry, these can be very sensitive and linear=
> over a range of several hundred microns.
>However, they are difficult to construct and have a working gap of less=20
>than a millimeter, and are prone to off axis sensitivity. They usually=20
>operate at relatively high frequencies, from khz to mhz, which complicates=
>design and implementation problems"
>It looks like the AD device would solve some of these concerns.

I agree.

>He then goes on to describe the VRDT. I suppose for the VBB sensor this=20
>would greatly simplify the electronic design if one can deal with small=20
>sensor gaps.

Biggest VRDT problem seems to be its low drive frequency.  In a feedback=20
design the large demod filters are prime contributors to loop oscillation=20

>I'm not sure about the noise. Does the VBB measure displacements in the 1=
>=B1 nm range?

With the sensor plates above, 1LSB=3D0.08nm.  But I think noise is what=20
determines the useful resolution.  However 0.3nm / sqrt-Hz and 2.1nm RMS at=
50 SPS isn't too shabby.  It would be interesting to assume a seismic-mass=
system and model how this would compare with commercial instruments and=20
earth-noise models.  I'm betting it won't look so bad.

>--- Just thinking out loud. I think it greatly depends on what type of=20
>sensing one wants to do local,regional or teleseismic. I like the=20

It would be gread to be able to use this with feedback.  No question that=20
you could use it for integral feedback, which mainly works below the low=20
frequency rolloff (say, below 0.011 Hz).  You could make an awesome=20
integrator by digital summing, then feed back with a D/A.  Mid frequency=20
range (derivative feedback) may not be practical.  Although you could keep=
your beam well centered with integral feedback, without derivative feedback=
at higher frequencies, you're limited by the +/- 1.0mm (+/- 0.5 mm max, for=
linearity) sensor gap.  I'm suspecting that clipping levels in the=20
mid-frequency range are going to be the biggest limitation.

Many thanks for your inputs,



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