How to measure the phase of an opamp?

[teraze]

I want to measure the open loop differential opamp's phase
so I type the expression: phase((1 / (VF("/inp") - VF("/inn"))/ VF("/Vout")))
But spectre show error like this:
*Error* Evaluating expression (phase((1 / (VF("/inp") - VF("/inn"))/ VF("/Vout")))).
*Error* ("difference" 11 t nil ("*Error* difference: can't handle (nil - nil)"))

I don't know what does it mean,
would anyone please express what's wrong,thank you

Teraze,

One option would be to use Spectre's stability analysis
which directly calculates the loop gain and phase for the
circuit. There is also some very good tutorial material on
testbench design and verification methodology, please
review the following material:

www.cadence.com/whitepapers/FVofDiffOpAmp_wp.pdf
http://www.ewh.ieee.org/r6/scv/cas/T...AS_Apr2004.pdf


Best Regards,


Brand Sheldon