The SBRC-190: A Cryogenic Multiplexer for Moderate-Background FIR Astronomy

The SBRC 190, a cryogenic multiplexer developed for far-infrared (FIR) photoconductor detectors operating at moderate backgrounds, is described. The circuit is based on the 32-channel CRC 696 CMOS device used on SIRTF. For applications such as encountered on SOFIA or Herschel, the new device permits higher backgrounds, a wider range of backgrounds, faster sampling, and enhanced synchronization of sampling with chopping. A relationship between sampling efficiency and noise requirements needed to achieve background-limited instrument (BLIP) performance is derived. Major design differences relative to the CRC 696 which have been incorporated in the SBRC 190 are: (a) an AC coupled, capacitive-feedback transimpedance unit cell, to minimize input offset effects, thereby enabling low detector biases, (b) selectable feedback capacitors to enable operation over a wide range of backgrounds, and (c) clamp and sample & hold output circuits to improve sampling efficiency, which is a concern at the high readout rates required. We discuss the requirements, design, fabrication issues, and expected performance.