Help explain the capacitance definition in Spice

[anhnha]

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)

 

[berkeman]

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...?