Geoffrey_Coram
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If you're manually adding up Cxy, you need to pay attention to the definition as Cxy = -dQx/dVy, see the minus sign.
Spectre (and other simulators) support a CAPTAB option that can print the total capacitance at a node, but this will give misleading numbers if there is a small drain resistor (you have to find the capacitance on the internal drain of M2).
Another method is to put a small-signal source on the drain: i2 (0 m2_D) isource dc=0 mag=1
then measure the small-signal (ac) voltage, and compute: Ceff = imag(1/vac(m2_D)) / (2 * pi * freq)
where "freq" is the frequency where you measured the ac voltage.
This is because I = Y V, where the admittance Y is the parallel combination of the effective conductance and capacitance: Y = Geff + j w Ceff I = 1 (mag=1 on source i2) Thus, Y = 1/V, and imag(1/V) = imag(Y) = w Ceff, so Ceff = 1/ (2 * pi * freq) * imag(1/V)
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