In this talk, I will present our recent papers about atomic Ramsey interferometer utilizing linear or nonlinear phase accumulations. Considering firstly a linear (Mach-Zehnder or Ramsey) interferometer, it has been shown that phase-squeezed state proposed by Berry and Wiseman exhibits the best sensitivity approaching to the Heisenberg limit . Similar with the Berry and Wiseman's state, we find that two kinds of phase-squeezed states can be generated dynamically with atomic Bose-Einstein condensates confined in a symmetric double-well potential, which shows the squeezing along spin operator Sy and the anti-squeezing along Sz, leading to sub-shot-noise phase estimation. Next, we consider a nonlinear interferometer with a product coherent spin state. Including collisional decoherence explicitly, phase sensitivity for estimating effective scattering strength of a two-component Bose-Einstein condensate is derived analytically. With a measurement of spin operator Sy, we confirm that the optimal sensitivity can reach super-Heisenberg limit.