Design and performance of a low noise circuit for VLWIR HgCdTe photoconductive detectors

Abstract

Due to VLWIR (very long wavelength infrared) signals are often very weak, it is about 1uV, little disturb can affect the performance of the total detector system very much. In order to achieve high Signal-to-Noise ratio, it is expected that the circuit can be designed to work as close as to the HgCdTe IR detector. That is to say the circuit can work normally at low temperature 77K even more low. On the other hand, according to the characteristics of a Very Long Wavelength HgCdTe photoconductive detector, its resistance is about 25Ω~50Ω, CMOS circuit for this low resistance is very difficult. In this paper, A new kind of circuit for this low resistance detector is designed. The operation principle and noise of the circuit are analyzed. The noise model of the circuit is given. An expression for its equivalent input noise is derived. This circuit for long wave photoconductive detector was implemented in 0.5μm CMOS process. The size of twenty-cell chip is 3mm×4mm and its noise performance is tested. The test result indicate that this circuit can work normally at low temperature 77K, the equivalent input noise is less than 1uV. This circuit is suit to many kinds of low resistance detector. The voltage gain is more than 10000. The linearity has been reached 90%. Finally, it can work normally either by ±2 or by ±1.5 voltage power supply. The bandwidth is more than 5Khz.