Multi-pixel readout of TES calorimeters

We are developing a superconducting Transition-Edge Sensor (TES) calorimeter array. We adopt calorimeter multiplex in frequency domain to read signals from the calorimeter array with a small number of front-end electronics and wirings. We further utilize Calorimeter Bridge Biased by an AC Generator (CABBAGE) approach^1,2 to eliminate the AC carrier in the output. We tested the method using a TES calorimeter, which has a transition temperature of 390 mK. Because of the high operating temperature, energy resolution (ΔE) of the calorimeter is limited to 200 eV at 5.9 keV even when it is biased with a DC current. We operated the calorimeter in CABBAGE circuit with 30 kHz sinusoidal bias and obtained ΔE of 250 eV. We found that there remains a small-amplitude residual in the output even at the bridge balance point. The residual contains not only 30 kHz component but also odd-order harmonics. We consider that this is due to the variation of the TES resistor with the bias current. Some of the degradation of ΔE from DC bias to AC bias can be explained by the fact that some of signal power is carried in the odd-order harmonics, which we did not utilize in the data reduction process. We also succeeded in operate the CABBAGE by 100 kHz, although ΔE was degraded to 380 eV because of unstable bath temperature during the data acquisition and probably because of low response of the signal readout circuit at the frequency.