Do any guys know what's the principle of cutting AC loop?

[castrader]

I want to know where to cut the loop to do a AC simulation?
I cut the LDO loop in same point with AC analysis and stb analysis of spectre seperately, but find the different result, obviously, the stb results is more acceptable, I want to know why my AC analysis is failed?

 

[leo_o2]

With loop breaking, you can see some cap is reduced at the left or the right of the breaking point.
The reason is STB will add cap and impedence back after the loop is broken. So STB is more accurate.

 

[lladnar23]

Essentially, as leo_o2 says, when you break the loop, you change the impedances seen at the break points. Unless you insert dummy impedances, or unless you break at an impedance close to zero or infinity, you could get different results.