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I have a lm386 circuit that requires pin 8, gain8, to be disconnected in the circuit.

So how do I write the spice subcircuit to account for no connection in the 8th pin?

Example

* IC pins:     1   2   3   4   5   6   7   8
*              |   |   |   |   |   |   |   |
.subckt lm386 g1  inn inp gnd out  vs byp g8
X_U1           0   2   0   3   2   0   4  ???  lm386 
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    \$\begingroup\$ Connect it to a node that isn't used for anything else. \$\endgroup\$
    – Dave Tweed
    Commented Mar 4, 2016 at 13:10
  • \$\begingroup\$ Thanks! I also added a 900M Ohm to ground and Vss and it still worked as well. \$\endgroup\$
    – daemondave
    Commented Mar 4, 2016 at 15:33

1 Answer 1

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A good way to use spice is to add "parasitics" that have real physical values, like that of air as you have done. Another way this can be useful is to add small caps to pins that represent the PCB capacitance of a couple of pF's. (the .cshunt parameter places 1e-15 caps on every pin to ground to improve stablitity, this is done automatically unless it is disabled). Be careful because the solver has a "dynamic range" and using very high and very low values can affect numerical stability of the solver. Adding parasitics is kind of an art in knowing when and where to palce them, but based on physical "real world" values. Everything in the real world has some kind of capacitance, inductance and resistance.

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