[gnramires]

OPamps, resistors, (switched) capacitors (and optionally inductors, although I believe they're infeasible to integrate) provide universal linear elements allowing the (approximate) reproduction of any linear system. Usually analog stages are linear, so that's not a huge issue. The key here is approximate: in reality, analog systems are precision-critical, have demands like low noise and subtle nonlinearities that apparently (per the article) make integration of an analog network not so attractive (because you can't control those parameters so easily).