Effect of Stray Impedance in Frequency-Division Multiplexed Readout of TES Sensors in POLARBEAR-2b

POLARBEAR-2b (PB-2b) is the second of three cryogenic receivers of the Simons Array cosmic microwave background polarization experiment. PB-2b contains over 7500 transition-edge sensor (TES) bolometers cooled to 250 mK and read out using digital frequency-division multiplexing (DfMux). Stray impedance in the DfMux circuit obscures TES characterization and affects TES dynamic behavior. In order to accurately characterize TESs, it is necessary to account for stray impedance in the bias circuit. We define a stray impedance model, and we describe the technique used to measure model parameters in situ and to remove their effects on TES characterization. We use the same model to predict TES dynamic behavior and show good agreement between data and the model.