Design and implementation of a single slope ADC for digital output cooled infrared readout integrated circuits

Thesis Type: Postgraduate

Institution Of The Thesis: Orta Doğu Teknik Üniversitesi, Faculty of Engineering, Department of Electrical and Electronics Engineering, Turkey

Approval Date: 2016


Supervisor: BARIŞ BAYRAM


Readout Integrated Circuits (ROIC) have been commonly implemented with analog video output buffers throughout history. Image signals are converted into digital by an external ADC card, which is placed outside the Dewar. Although analog output method is easier to implement, it is susceptible to the environmental noise due to the non-differential output. Moreover, the ADC proximity card placed outside of the dewar contributes to the system complexity. This thesis presents the design of a single slope ADC dedicated to digital output readout integrated circuit for cooled infrared detectors. The objective is to develop an ADC stage integrated into ROIC which enables ROIC to have digital output. Digital output method isolates noise caused by out of ROIC mediums. At the system level, removal of the ADC proximity card reduces system complexity and volume of the IDDCA system which is important for avionic and missile applications. It also reduces the system cost associated with external ADC components. A digital output ROIC is fabricated using standard commercially available 0.18 μm CMOS technology. Column parallel topology is adopted as ADC placement method inside ROIC. ADC is cryogenic compatible which is essential for infrared systems. Cryogenic temperature causes significant parameter shifts for MOS transistors. Therefore, feedback based calibration circuit is implemented within ADC in order to prevent integral nonlinearity (INL) error. ADC is guaranteed to be monotonic because of the selected single slope ADC architecture. Implemented design is suitable for 15 μm pixel pitch imaging applications. The dimensions of the column ADC and shared circuitry are 370 μm x 30 μm and 660 μm x 540 μm, respectively. Input signal swing range of the ADC is 2 V, and the implemented design includes an out of swing range recovery circuit in case of very bright conditions. ADC has been functionally verified and infrared images are sampled with the fabricated ROIC and a LWIR detector. Implemented ADC has 12 bits of resolution and supports 27 kHz sampling speed. It also has programmable high speed mode that extends conversion speed to more than 100 kHz with 10 bits resolution or 50 kHz with 11 bits resolution. Simulated SNR performances of the ROIC are 73.7 dB at cryogenic temperature and 71.22 dB at room temperature. Measured noise count is less than 1 LSB RMS in all cases. Power consumption is less than 40 μW per column ADC which makes digital output ROIC feasible compared to its analog output counterpart.