Help explain the capacitance definition in Spice

AI Thread Summary
In SPICE, capacitance is defined as Cxy = -dQx/dVy for x ≠ y and Cxx = dQx/dVx, which reflects the relationship between charge and voltage changes. The user expresses confusion about this definition, noting that it seems different from the standard capacitance formula C = dQ/dV. Another participant questions the source of this definition, suggesting it may not align with typical SPICE models and could resemble those from other simulation software like ANSYS. The discussion highlights a need for clarification on how capacitance is represented in different simulation contexts. Understanding these definitions is crucial for accurate circuit modeling in SPICE.
anhnha
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Hi,
In Spice, the capacitance between two points x, y is defined as follows:
Cxy = - dQx / dVy for x != y
Cxx = dQx/dVx

Could anyone help me explain why it is defined like that?
I know that C = dQ/dV but the formula above seems strange to me.

Is my understanding below correct?
Cxy = dQx/d(Vx - Vy)
 
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anhnha said:
Hi,
In Spice, the capacitance between two points x, y is defined as follows:
Cxy = - dQx / dVy for x != y
Cxx = dQx/dVx

Could anyone help me explain why it is defined like that?
I know that C = dQ/dV but the formula above seems strange to me.

Is my understanding below correct?
Cxy = dQx/d(Vx - Vy)

Could you say more about where you are seeing this in SPICE? Which version of SPICE?

I'll check my SPICE Model Definition Textbook tomorrow at work, but that doesn't ring any bells at the moment. It looks more like an ANSYS or other physical simulator model...?
 
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