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LTspice Zener Models

Started by Rick C March 7, 2020
I'm looking to model a circuit with a zener diode in it and the exact part doesn't seem to have a simulation model.  Looking at the 11 choices in LTspice I see a variety of parameters.  The one that I'd probably want to match the most closely is the source resistance, Rs.  This seems to be a very difficult number to pin down. 

The part in question is CZRU52C6V2 by Comchip.  They list 10 ohms at 5 mA in the table.  But they also provide a graph of dynamic impedance vs. "normall" zener voltage at three currents.  This graph looks like the terrain profile of a trail hike!  It seems to be amazingly sensitive to the zener voltage.  Using my operating current of 1.23 mA and interpolating on the graph I get about 18-20 ohms impedance.  

Now the problem becomes equating that to a model.  They LTspice models show an Rs parameter.  Can I simply look for a model with a specified Rs close to 18 ohms?  Or is this parameter massaged in some way to get the Rs at any given operating point?  Seems the largest value in the LTspice models is 0.5 ohms, so not very close.  

I don't know how important it is to get this "right".  The zener is used to set the bias voltage for an opamp circuit and several points in the circuit use it.  Two channels use separate zener circuits to minimize crosstalk.  I'm more interested in how any leaked signal between stages will impact frequency response, but I suspect that will be very minor.  I would like to actually know for sure though. 

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  Rick C.

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