This paper presents a novel architecture of an impedance spectrometer dedicated to the characterization and diagnostics of large fuel cell stacks operated in galvanostatic mode. The validation tests are first performed on a single proton exchange membrane fuel cell (PEMFC). Then, experiments are carried out on a 20-cell PEMFC stack delivering more significant power levels. The proposed impedancemeter allows spectrum measurements on cells located in the middle of the stack, where common mode potentials are usually too high for commercial devices. Moreover, the impedances of different individual cells in the stack are acquired with a synchronous measurement reference (global stack impedance). This capability allows distinguishing any singular cell behavior or drift effect of operational parameters (e.g., stack temperature and polarization current).