Developments in time-division multiplexing of X-ray transition-edge sensors.
William B. Doriese, Kelsey M. Morgan, Douglas A. Bennett, Edward V. Denison, Colin P. Fitzgerald, Joseph W. Fowler, Johnathon D. Gard, James P. Hays-Wehle, Gene C. Hilton, Kent D. Irwin, Young Il Joe, John A. Mates, Galen C. O'Neil, Carl D. Reintsema, Nigel O. Robbins, Daniel R. Schmidt, Daniel S. Swetz, Hideyuki Tatsuno, Joel N. Ullom, Leila R. Vale
Time-division multiplexing (TDM) is a mature scheme for the readout of transition-edge sensors. Variants of TDM, which is based on superconducting-quantum-interference-device (SQUID) current amplifiers, have to date been field-deployed in several microcalorimetric (X-ray and gamma-ray) spectrometers of the 250-pixel scale and several bolometric (sub-mm and microwave) astronomical cameras of the multiple-kilopixel scale. The important characteristics of TDM are that the TESs are dc-biased, rows of SQUIDs are switched on one at a time, columns of SQUIDs are read out in parallel, and each TES is maintained in its own digital-feedback loop (to keep the sinusoidal SQUID-responsive curve in a linear range). Here we present the details of two different versions of our TDM system to read out X-ray TESs. The first, which has been field-deployed in six X-ray spectrometers of either 160~pixels (8~columns $\times$ 20~rows) or 240~pixels (8~columns $\times$ 30~rows), has a three-SQUID-stage architecture, switches rows every 320~ns, and has total readout noise of 0.41~\microphi \ (non-multiplexed; referred to the first-stage SQUID). The second, which is presently under development, has a two-SQUID-stage architecture, switches rows every 160~ns, and has total readout noise of 0.19~\microphi \ (non-multiplexed; referred to the first-stage SQUID). In a demonstration of this new architecture, a 1-column $\times$ 32-row array of NIST TESs achieved average energy resolution of 2.55$\pm$0.01 at 6~keV when observing fluorescent X-rays from a Mn target.