Noise Modeling and Calibration of a Two-Stage Cryogenic Charge Amplifier for the SPLENDOR Experiment

The SPLENDOR Collaboration studies novel narrow-gap semiconductors and engineered a substrate agnostic detector platform to achieve $\mathcal{O}$(meV) energy sensitivity designed for low mass dark matter searches. This was achieved using low-capacitance and low-noise commercial CryoHEMTs in a split-stage topology integrated throughout a dilution refrigerator. Designed with a source-follower HEMT at the base temperature stage and a voltage amplifier at 4,K, this amplifier has input-limited voltage noise of 10 $\text{nV}\mathrm{/}\sqrt{\text{Hz}}$ and current noise of 100 $\text{aA}\mathrm{/}\sqrt{\text{Hz}}$ at 1kHz. In agreement with this noise level and a photon calibration, this amplifier has a $\text{19}\pm \text{4}$ electron resolution.