  Frequency domain multiplexing (FDM) is the baseline readout system for the
X-ray Integral Field Unit (X-IFU) on board the Athena mission. Under the FDM
scheme, TESs are coupled to a passive LC filter and biased with alternating
current (AC bias) at MHz frequencies. Using high-quality factor LC filters and
room temperature electronics developed at SRON and low-noise two-stage SQUID
amplifiers provided by VTT, we have recently demonstrated good performance with
the FDM readout of Mo/Au TES calorimeters with Au/Bi absorbers. We have
achieved a performance requested for the demonstration model (DM) with the
single pixel AC bias ($\Delta E=$1.8 eV) and 9 pixel multiplexing ($\Delta
E=$2.6 eV) modes. We have also demonstrated 14-pixel multiplexing with an
average energy resolution of 3.3 eV, which is limited by non-fundamental issues
related to FDM readout in our lab setup.
