At 12:36 AM 10/10/2012, you wrote:

>On Tue, 2012-10-09 at 22:28 -0400, Brett Nordgren wrote:
> > Hi Bob,
> >
> > The AD706 is quite a low-noise device.  Typically 15E-9 V/sqrt-Hz
> > with 1/f corner at 4Hz and 50E-15 A/sqrt-Hz with 1/f corner at 8Hz.
> >
> > At lowest frequencies the LT1007A looks like it typically has 12x
> > lower voltage noise and 400x higher current noise than the 706.  I
> > need to look at how it would work in some of the higher-impedance
> > locations of our circuits.
>
> > At this point the broadband designs generally have lower noise than
> > the local background, though at Dave's excellent site that may not be
> > entirely true, below about 20 mHz.  What I really like to know is
> > which parts are generating how much noise and why.  So far the
> > position sensor has resisted noise modeling.  Measurements are in order.
>
>The program CCICAP has a Noise Table feature that can tell you exactly
>that.  See the noise table for the design I am experimenting with:

Hi Bob,

My problem is that Dave's excellent displacement sensor circuit in 
the force-balance verticals uses a capacitance half-bridge sensed by 
a switched-capacitor charge-transfer circuit.  In practice its noise 
has been extremely low, but I need to learn more before I can 
adequately model it.  The possible noise sources that I can think of 
are kT/C noise, clock jitter, and small variations in switch charge 
injection.  These don't model so nicely, hence my interest in making 
the measurements.  Noise modeling the rest of the system has not been 
a problem so far.

Brett 


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