  Digital Frequency-Domain Multiplexing (DfMux) is a technique that uses MHz
superconducting resonators and Superconducting Quantum Interference Device
(SQUID) arrays to read out sets of Transition Edge Sensors. DfMux has been used
by several Cosmic Microwave Background experiments, including most recently
POLARBEAR-2 and SPT-3G with multiplexing factors as high as 68, and is the
baseline readout technology for the planned satellite mission LiteBIRD. Here,
we present recent work focused on improving DfMux readout noise, reducing
parasitic impedance, and improving sensor operation. We have achieved a
substantial reduction in stray impedance by integrating the sensors,
resonators, and SQUID array onto a single carrier board operated at 250 mK.
This also drastically simplifies the packaging of the cryogenic components and
leads to better-controlled crosstalk. We demonstrate a low readout noise level
of 8.6 pA/Hz$^{-1/2}$, which was made possible by operating the SQUID array at
a reduced temperature and with a low dynamic impedance. This is a factor of two
improvement compared to the achieved readout noise level in currently operating
Cosmic Microwave Background experiments using DfMux and represents a critical
step toward maturation of the technology for the next generation of
instruments.
