  The third generation South Pole Telescope camera (SPT-3G) improves upon its
predecessor (SPTpol) by an order of magnitude increase in detectors on the
focal plane. The technology used to read out and control these detectors,
digital frequency-domain multiplexing (DfMUX), is conceptually the same as used
for SPTpol, but extended to accommodate more detectors. A nearly 5x expansion
in the readout operating bandwidth has enabled the use of this large focal
plane, and SPT-3G performance meets the forecasting targets relevant to its
science objectives. However, the electrical dynamics of the higher-bandwidth
readout differ from predictions based on models of the SPTpol system. To
address this, we present an updated derivation for electrical crosstalk in
higher-bandwidth DfMUX systems, and identify two previously uncharacterized
contributions to readout noise. The updated crosstalk and noise models
successfully describe the measured crosstalk and readout noise performance of
SPT-3G, and suggest improvements to the readout system for future experiments
using DfMUX, such as the LiteBIRD space telescope.
