Yuma2Models.zip  ReadMe.txt

These files were created using Linear Technology's free Spice modeling program, 'LTSpice'.  Originally known as 'Scad' and intended for modeling switching regulators, it has evolved into a powerful general-purpose implementation of Spice which may be downloaded at   http://www.linear.com/designtools/software/#LTspice  

First, install LTspice.  Its default installation folder will be 'C:\LTC\LTspiceIV\', which apparently was used to avoid possible issues with Windows' User Account Control.  

To run the models, first extract these zipped files into their own folder.  Then look in it for the FBV library file.  If it is named 'FBV.libx' change its file extension to '.lib'.  It may have been changed to allow sending these as attachments via the e-mail providers who do not accept '.lib' and certain other file extensions in attachments.  

From there, any of the four schematic, '.asc' files may be opened in the LTSpice simulator.  'Yuma2 Gain' allows plotting of the instrument transfer function and instrument clipping levels.  'Yuma2 Loop Gain' models the instrument feedback Loop Gain.  For doing noise modeling there are two schematics which may be useful. 'Yuma2 Noise.asc' is the normal noise simulation and 'Yuma2 Noise0.asc' is the same file with all noise-generating components set for zero noise.  This allows one to more easily evaluate the noise contribution from each element individually.  Also included, in the 'netlist' folder, are four corresponding, '.net' files, which might be useable with other Spice versions.  If the text on the schematic appears too large or too small, go to 'Tools' > 'Control Panel' > 'Drafting Options' and in the 'Font Properties' section, adjust 'Size' to correct the problem.
 
The parameters and schematics were created to model the Yuma2, though no attempt was made to define the mass values, sensor gain or force transducer constant with much accuracy.  Those may be adjusted to match a specific instrument, by editing the appropriate '.param' lines on the schematics.

While the desired schematic is being displayed, to run the simulation, select Simulate > Run

The results plot will be shown in a window displaying the corresponding '.raw' results file.  To select which curves are displayed and adjust the plot axes and text you may want to apply the appropriate plot settings, '.plt' file.  Select the plot window, then PlotSettings > Open Plot Settings File > [desired '.plt' file].  That will select the plot traces and set their scales and comments to what was originally intended.  After running the simulation, you are also free to examine the voltage, current, or noise contribution of any other circuit elements.

The noise model for the AD706 op-amp was created by starting with a simple noiseless model and then explicitly adding the expected voltage and current noise parameters from the AD706 spec sheet curves, "en=15nV enk=4Hz in=50fA ink=8Hz".  It has been noted by others that not all Spice op-amp models from the various manufacturers accurately model their noise.  Using my own model assures that the noise parameters will be what I expect.

The simplified displacement sensor parameters, Rsrc and Ct, were chosen to make its base band response match the behavior observed with a detailed Spice transient analysis of that circuit.  Those values may be affected by the sensor plate areas and spacings, and more analyses are needed for the various instruments.  No consideration was given to other possible sensor noise sources such as fluctuations in the switch charge-injection, supply voltages or clock phase jitter.  Although they are not likely to be significant effects, their noise contributions are, as yet, undetermined.