Geoffrey_Coram
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You've probably got a poorly-defined model. What happens in subthreshold? If you don't have a GMIN or subthreshold current equation, then you'd likely get a zero diagonal message. The Jacobian (matrix) is loaded with partial derivatives of all the currents with respect to the node voltages. If there's a zero diagonal at nodeX, then it means that, when linearized about the operating point, changing the voltage on nodeX will not affect the current into nodeX.
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