  Advanced ACTPol is an instrument upgrade for the six-meter Atacama Cosmology
Telescope (ACT) designed to measure the cosmic microwave background (CMB)
temperature and polarization with arcminute-scale angular resolution. To
achieve its science goals, Advanced ACTPol utilizes a larger readout
multiplexing factor than any previous CMB experiment to measure detector arrays
with approximately two thousand transition-edge sensor (TES) bolometers in each
150 mm detector wafer. We present the implementation and testing of the
Advanced ACTPol time-division multiplexing readout architecture with a 64-row
multiplexing factor. This includes testing of individual multichroic detector
pixels and superconducting quantum interference device (SQUID) multiplexing
chips as well as testing and optimizing of the integrated readout electronics.
In particular, we describe the new automated multiplexing SQUID tuning
procedure developed to select and optimize the thousands of SQUID parameters
required to readout each Advanced ACTPol array. The multichroic detector pixels
in each array use separate channels for each polarization and each of the two
frequencies, such that four TESes must be read out per pixel. Challenges
addressed include doubling the number of detectors per multiplexed readout
channel compared to ACTPol and optimizing the Nyquist inductance to minimize
detector and SQUID noise aliasing.
