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Cryogenic readouts are the critical electronics for infrared (IR) detector arrays and, in most instances, the dominant source of noise in an IR detection system. Various designs have been developed and perfected over years to minimize the read noise and optimize the circuitry for particular detector arrays for which the readouts are intended to be used. The present effort in this area--to develop readouts that operate at 2 kelvin and achieve improved sensitivity--is driven by the NASA charter to provide state-of-the-art IR technol-ogy to the astronomical community at large.
The simplest unit-cell design is the source follower per detector, which employs a single metal-oxide-semiconductor field-effect transistor (MOSFET) in the source follower configuration to read the detector signal. Although the simplicity of this design is attractive in terms of fabrication, minimal use of electronics real estate, and operation, it suffers from the inherent drawback that the integrated charge at the MOSFET gate debiases the detector and thereby degrades the detector linearity. In extreme cases, it can significantly diminish the detector photocurrent. Detector debiasing could pose severe limitations for detector arrays that require low bias levels, such as those used in the far-IR. A capacitive trans-impedance amplifier (CTIA) is one possible solution. This design uses a transistor in an amplifier mode and includes a capacitor in the feedback loop. The feedback capacitor serves as the integrator and, by virtue of the negative feedback, pins the detector node to a constant voltage.
The first generation of CTIA readouts, CRC-696, was manufactured by Hughes/Santa Barbara Research Center for the multiband infrared photometer (MIPS) for the Space Infrared Telescope Facility (SIRTF) instrument. This device was a 1 x 32, single-gain, DC-coupled multiplexer, optimized for low photocurrents. A few of these devices, with and without IR detectors, were tested in the lab at Ames. The success of the CRC-696 led to the development of the next generation of these devices--the SB-190--with added features to expand and enhance their performance. The SB-190 is a 1 x 32, multigain, AC-coupled CTIA designed to accommodate a broad range of IR flux (including photocurrent levels consistent with airborne astronomy applications) (fig. 1). The multiple gain is achieved by having several integrating capacitors in the feedback loop that can be enabled in eight possible combinations.
These devices have been designed, developed, and characterized; their first application is intended to be for the far-IR detector array of the airborne infrared echelle spectrometer (AIRES), a facility instrument for the Stratospheric Observatory for Infrared Astronomy (SOFIA). To date, several of these devices have been tested; they showed adequate performance, although better performing devices are expected when the processing is refined and better controlled. The typical read-noise at the highest gain setting and with corre-lated double sampling is about 250 electrons. Further tests are under way, and this readout will be integrated with a 1 x 24 prototype Ge:Sb array in the near future.
Point of Contact: J. Farhoomand
(650) 604-3412
jfarhoomand@mail.arc.nasa.gov
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Fig. 1. A prototype 1 x 32 SB-190 multiplexer bonded to 4 Ge:Ga far-infrared detectors.
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