Most extractors correlate pretty well when it comes to getting an accurate representation of the power network. (in terms of R’s and C’s)

While extraction in most tools is pretty comparable and accurate, since the size of the extracted networks is huge, a Big differentiator is the algorithm used to actually calculate V= IR (for static) or I = cdv/dt + GV (for transient).

Since we would model all the gates/macro’s as current sources post some power analysis iteration (post which the current sinks are known), the equation would involve solving
I = GV. (G is the conductance matrix). This is an inverse of a huge resistance matrix (resistance matrix ia arrived at after extraction) and then multiplication of matrices.

So essentially to find the V’s in the network (to calcuate IR drop) would involve solving a huge number of equations best modelled using a sparse linear matrix. Here some approximations are used and iterative techniques are usually employed (for ex: the Conjugate Gradient method) as opposed to Gauss Jordan or Guas seidel which can be too complex or some other krylov sub space technique could be used.

Depending on the numerical technique used to solve the Inverse/Multiplication of these huge sparse linear matrices, the IR drop resuts can vary significantly across tools.

The analysis is more complex in case of transient IR drop analysis and further gets complicated if inductances are also extracted as the matrices which result post extraction require a lot of post processing to suit any of the fast iterative techniques to be applicable on them.