A methodology for designing highly accurate readout circuits for infrared (IR) image detectors that have large pixel detector capacitances (of the order of tens of picofarads) and are required to operate with short exposure times (<100 µs) has been devised. In this context, "highly accurate" signifies capable of (1) nearly linear response over a wide dynamic range with (2) little or no image lag ("ghost" image readout attributable to capacitive retention of charge from preceding image frames), and (3) low readout noise. The methodology has been used to enhance the performance of NASA's Airborne Visible/Infrared Imaging Spectrometer (AVIRIS), and is expected to satisfy an increasing need for highly accurate readout circuits in other applications, including other imaging spectrometers and infrared video cameras now undergoing development.
The methodology improves upon the prior art in two main ways:
- It provides for refinement of the design of pixel amplifiers and other pixel readout circuitry on the basis of understanding gained through analysis of previously neglected second-order electronic effects - including notably image lag related to abrupt transitions of signal currents.
- It incorporates the concept that, at least in principle, it should be possible to derive an algorithm to correct for image lag.
The readout circuitry for each pixel, as contemplated in this methodology, is based on that of the AVIRIS. This circuitry (see figure) features a conventional buffered-direct-injection (BDI) input circuit followed by amplifier stages of optimized design. Similar BDI readout circuits designed according to older methodology generate residual image signals when bright images are followed by dark images, giving rise to errors in the estimates of the dark images; and when dark images are followed by bright images, responses are oscillatory, giving rise to errors in estimates of the bright images. The errors are functions of time and detector capacitances.
The analysis of second-order effects yields closed-form expressions for response times for both low-to-high and high-to-low transitions. These expressions are what make it possible to optimize pixel amplifier design and to choose appropriate feedback capacitors to minimize circuit error and image lag. In an initial application of this part of the methodology to a linear array of photodetectors for the AVIRIS, it was found that the signal-to-noise ratio was increased by a factor of 2 to 3, relative to older designs, and that image lag was reduced to less than 10 percent over the entire dynamic range of pixel signal current from 10 pA to 10 nA. The measured input-referred noise was found to be less than 300 electrons.
A first-order analysis has been performed in an initial effort to develop software to compensate for image lag. This analysis yields recursive equations that can be used to estimate and correct for errors. Software that implements these equations has been tested on AVIRIS readout data and found to reduce errors but also to occasionally introduce new errors. The inability to eliminate all errors has been attributed to inadequate mathematical modeling of circuit behavior, including inaccuracies in estimates of response times. To achieve greater accuracy, it would be necessary to derive equations of greater complexity, based on a second-order analysis. Of course, it would be more difficult to implement such equations in software.
This work was done by Bedabrata Pain of Caltech for NASA's Jet Propulsion Laboratory.
NPO-20522
This Brief includes a Technical Support Package (TSP).
Readout for Fast IR Imaging With Large Detector Capacitance
(reference NPO20522) is currently available for download from the TSP library.
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Overview
The document presents a technical support package from NASA focusing on advancements in readout circuits for infrared (IR) image detectors, particularly in the context of the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS). The primary challenge addressed is the need for highly accurate readout circuits that can handle large pixel detector capacitances (tens of picofarads) while operating with short exposure times (less than 100 microseconds).
The methodology introduced aims to achieve a nearly linear response over a wide dynamic range, minimize image lag (ghost images from previous frames), and reduce readout noise. This is crucial for applications in airborne spectrometry, where the combination of large capacitance and short exposure times can lead to significant accuracy issues. The document outlines a new design methodology that optimizes pixel amplifier design and feedback capacitors to enhance performance.
Key improvements include a high-speed, low-noise, wide dynamic range linear multiplexer array for readout, which has shown a 2-3 times increase in signal-to-noise ratio and a reduction of image lag to less than 10% across a dynamic range of 10 pA to 10 nA. The design also incorporates a software error correction algorithm to address issues related to abrupt signal transitions, which can cause inaccuracies in image readings.
The document emphasizes the importance of understanding the response time of the readout circuits as a function of circuit parameters, allowing designers to optimize performance for both high-to-low and low-to-high signal transitions. This is particularly relevant for applications that require rapid and accurate imaging, such as environmental monitoring, geological surveys, and urban infrastructure analysis.
Overall, the advancements detailed in this document represent a significant leap in the capabilities of infrared imaging systems, enabling more precise data collection and analysis in various scientific and commercial applications. The work was conducted at the Jet Propulsion Laboratory under NASA's auspices, highlighting the ongoing collaboration between government and research institutions to push the boundaries of technology in remote sensing and imaging.
