  We are developing the frequency domain multiplexing (FDM) read-out of
transition-edge sensor (TES) microcalorimeters for the X-ray Integral Field
Unit (X-IFU) instrument on board of the future European X-Ray observatory
Athena. The X-IFU instrument consists of an array of $\sim$3840 TESs with a
high quantum efficiency ($>$90 \%) and spectral resolution $\Delta E$=2.5 eV
$@$ 7 keV ($E/\Delta E\sim$2800). FDM is currently the baseline readout system
for the X-IFU instrument. 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. An integrated
noise equivalent power resolution of about 2.0 eV at 1.7 MHz has been
demonstrated with a pixel from a new TES array from NASA/Goddard (GSFC-A2). We
have achieved X-ray energy resolutions $\sim$2.5 eV at AC bias frequency at 1.7
MHz in the single pixel read-out. We have also demonstrated for the first time
an X-ray energy resolution around 3.0 eV in a 6 pixel FDM read-out with TES
array (GSFC-A1). In this paper we report on the single pixel performance of
these microcalorimeters under MHz AC bias, and further results of the
performance of these pixels under FDM.
