Noise properties of preamplifier to be used with LN₂-cooled HgCdTe photodetector

Objectives. Photoresistors based on a solid solution of mercury–cadmium–tellurium (MCT) have been used in infrared (IR) technology for over 60 years. They can have a sensitivity range in the wavelength region from 1 μm to 15 μm, depending on Hg_1−xCd_xTe composition. The resistance of photosensitive MCT elements is (depending on their area) tens of Ohms, and for such a resistor the thermodynamically expected Nyquist noise is less than 1 nV/√Hz. Modern semiconductor technologies ensure a high level of quality of both photodetectors and input stages of integrated circuits for amplifying the signal from them. The aim of this work is to study the noise properties of the electronic unit developed for joint operation with a liquid nitrogen cooled MCT-photodetector.

Methods. An analog input-output digital signal processor card P25M (Innovative, Inc., USA) was used to measure and accumulate the noise spectra of the signal in the frequency range 0–1 MHz. The card has four 16-bit ADCs of sampling rate up to 25MSpS, a Spartan-3 field-programmable gate array controlling them, a TMS320C6713 processor, and RAM, in order to transmit the collected digital data to the motherboard through a common PCI-X slot. The spectra of the received data were calculated using the fast Fourier transform algorithm with subsequent averaging of the square of the amplitude for all spectral components.

Results. The noise properties of comparatively modern integrated circuits currently used for this task were considered. The noise density spectra of the first stage (ADA4898-2), the second stage (AD8034), and bias current sources (AD8397 and LT3009) were measured. It was found that the spectral density of the input noise of the operational amplifier ADA4898-2 is comparable to the Nyquist (thermodynamically expected) noise of a 20–100-Ohm resistor corresponding to the resistance of the photosensitive element. This means that the selected operational amplifier is ideal for resolving the technical problem discussed herein. Meanwhile, it was also established that the noise spectrum of the LT3009, ADR510 voltage and current stabilizer integrated circuits contains a noticeable drift component with a spectral density of “pink noise” 1/f ^α (f – frequency, α ≈ 1).

Conclusions. It was shown that the spectral noise density of the electronic components, reduced to the input of the device, is several times lower than the noise density of the photodetector used.

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