Akamatsu, H.Gottardi, L.van der Kuur, J.de Vries, C. P.Bruijn, M. P.Chervenak, J. A.Kiviranta, M.van den Linden, A. J.Jackson, B. D.Miniussi, AntoineRavensberg, K.Sakai, KazuhiroSmith, S. J.Wakeham, Nicholas2022-01-202022-01-202020-01-28Akamatsu, H., Gottardi, L., van der Kuur, J. et al. Progress in the Development of Frequency-Domain Multiplexing for the X-ray Integral Field Unit on Board the Athena Mission. J Low Temp Phys 199, 737–744 (2020). https://doi.org/10.1007/s10909-020-02351-3https://doi.org/10.1007/s10909-020-02351-3http://hdl.handle.net/11603/24035Frequency-domain multiplexing (FDM) is the baseline readout system for the X-ray Integral Field Unit on board the Athena mission. Under the FDM scheme, TESs are coupled to a passive LC flter and biased with alternating current (AC bias) at MHz frequencies. Using high-quality-factor LC flters and room-temperature electronics developed at SRON and low-noise two-stage SQUID amplifers 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 with the single-pixel AC bias (ΔE = 1.8 eV) and nine-pixel multiplexing (ΔE = 2.6 eV) modes. We have also demonstrated 14-pixel multiplexing with an average energy resolution of 3.3 eV, which is limited by nonfundamental issues related to FDM readout in our laboratory setup.8 pagesen-USThis work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.Public Domain Mark 1.0http://creativecommons.org/publicdomain/mark/1.0/Text