Cryogenic Integrated Offset Compensation for Time Domaine SQUID Multiplexing

International audienceSuperconducting QUantum Interference Device (SQUID) multiplexing is an important issue in the use of large arrays of superconducting bolometers (TES). A Time Domain Multiplexer (TDM) combines input TES signals into one output signal using several SQUIDs. Different TESs, SQUIDs and amplifiers characteristics induce unavoidable different offsets on the multiplexed signal. Moreover, assuming the periodicity of the SQUID characteristic, the Flux Locked Loop (FLL) operating point is defined at modulo Phi_0 which could also leads to a large output offset. In multiplexed mode, offsets resulting from different pixels induce a parasitic signals often larger than the TES one. This offset signal drastically constrains the readout dynamic range and thus the maximum gain allowed. It also limits the signal-to-noise ratio, the FLL stability and the multiplexing frequency. Offset in SQUID readout is discussed and offset compensation for TDM is presented. Simulation shows the dynamic calibration and compensation on a simplified 4:1 TDM. Dynamic offset compensation is being implemented on a cryogenic BiCMOS SiGe integrated circuit operated at 4K for 128:1 TDM