  Digital frequency multiplexing (dfMux) is a readout architecture for
transition edge sensor-based detector arrays and is used on telescopes
including SPT-3G, POLARBEAR-2, and LiteBIRD. Here, we present recent progress
and plans for development of a sub-Kelvin SQUID architecture for digital
frequency multiplexed bolometers. This scheme moves the SQUID from the 4 K
stage to the 250 mK stage, adjacent to the bolometers. Operating the SQUID on
the detector stage may offer lower noise and greater scalability. Electrical
performance will be improved as a result of decreased wiring length and reduced
parasitics, allowing for higher multiplexing factors and lower bolometer
R_normal . These performance improvements will enable ultra-large focal planes
for future instruments such as CMB-S4.
