Research Of Readout Integrated Circuit For VLWIR FPAs

Very long wavelength infrared focal plane array (VLWIR FPA) is a typicalrepresentative of the third generation of infrared Technology, and becomes a hotresearch field in the world. The readout integrated circuit (ROIC) used to pre-processthe weak photocurrent is an important part of the IR detection system. The mainfunctions of ROIC are chiefly consisted of signal integration, amplifying, filteringand sampling, noise reduction, and conversion of transmission types etc. At present,the VLWIR detectors have very low output impedance. In order to achieve highinjection efficiency, the BDI input circuit should have a low input resistance.Furthermore, the VLWIR detectors run at high background environment with a largeleakage current at the operation temperature of50K, so the integrator capacitor isvery easy to saturation, and it is difficult to get a high signal to noise ratio (SNR) forIRFPA. Due to the limitations of VLWIR FPAs’ features, it is a big challenge todesign a high performance ROIC.The ROICs for VLWIR FPAs have been analyzed and compared in this paper,and then improved structures are discussed theoretically, rationality and feasibility isalso analyzed. In our study, we designed a32×32ROIC for VLWIR detector, whichuses buffered direct injection (BDI) unit cell as input circuit to reduce the inputresistance, increase the injection efficiency of signal current, and provide precisebiasing voltage to the detector. Due to very low output impedance of VLWIRdetectors, a high gain feedback amplifier is used to provide the inverting gain by adifferential stage. By means of novel current mode background suppression, theintegration time is increased when VLWIR FPAs run at high backgroundenvironment. At the same time, better dynamic range is also achieved. The final chipis fabricated with HHNEC0.35μm1P4M process technology. After coupling toinfrared detector chip, the performance of MWIR FPA and VLWIR FPA have beenevaluated. The test results show that the ROIC can be used in VLWIR FPA, and all the performance parameters satisfy the design expectations and project requirementsat the temperature of50K.This paper have carried out a detailed analysis of the ROIC’s test results,studied the trends of MOS transistors’ parameters under the HHNEC0.35μmmanufacturing process and the impacts on the overall performance of ROIC.Meanwhile, the paper also analyzed the test results of VLWIR FPA, and proposedthe ideas at the view of circuit design point toimprove the overall performance ofIRFPA.The main Disadvantages of VLWIR FPAs found during testing have beensummarized. Furthermore, a novel ROIC has been designed for verifying itsfunctionality and performance. The new ROIC has the ability to operate with currentmode background suppression, which has ability to eliminate the background currentfor the unit itself, and it increases the integration time. To improve the linearity andprovide tight control of the detector bias, a shared buffer direct injection (SBDI) witha shared telescope op amplifier for2×2unit is used as input circuit. Meanwhile, theunity gain amplifier is selected to improve output swing and readout frequency.These circuits have feasibility and will be fundamentals for future work.