Two-Stage SQUID Amplifier for the Frequency Multiplexed Readout of the X-IFU X-Ray Camera

The X-ray Integral Field Unit (X-IFU) is a cryogenic X-ray camera intended for the ATHENA space observatory. Its focal plane will contain a kilopixel array of Transition Edge Sensors (TESs), originally intended to be read out by a Frequency Domain Multiplexer involving 40 carrier frequencies in the 1-5 MHz range per readout channel. We review dimensioning principles and fabrication of a SQUID tandem, operating at 50 mK and 2 K temperatures, intended to amplify the multiplexed signal from the TES array. The first measurements indicate a 40 ħ coupled energy resolution in the relevant frequency band, and sufficient linearity to reach the required 1% total harmonic distortion. Additionally, we describe the non-multiplexed amplifier SQUID for the X-IFU anti-coincidence detector.