Cryogenic readout electronics for astronomical applications.

IMEC has developed the cold readout electronics for the ISOPHOT experiment. For this low-background application, one uses custom designed highly sensitive integrating charge amplifiers for the readout of the extrinsic IR-detectors. The use of CMOS circuits allows the multiplexing and readout of a large number of detectors at the detector temperature, with a very low power dissipation. The CMOS readout amplifier/multiplexer for deep cryogenic operation is discussed. The device is able to interface directly with extrinsic photoconductive detectors cooled down to the 1.8 K-10 K range. In order to observe faint objects under low light level conditions the integration capacitor is 80 fF, featuring a saturation at 1 million charge carriers for an output voltage swing of 2 V and a noise level of 0.5 mV rms in non-destructive readout mode. With this curcuit, coupled to detectors with a responsivity of around 5-10 A/W, it is possible to reach NEP (noise equivalent power) values of 10^-17W/√(Hz). The multiplexer circuits can be operated with only 10 wires for the supplies, and the clocking of the circuits.