Complementary Metal Oxide Semiconductor Readout Integrated Circuit for Imaging Applications

Abstract

In this paper presents a novel continuous-time simultaneous-readout scheme for active imaging systems based on orthogonal modulation of photo detector signals. The superimposed-continuous-time approach presented here differs from the conventional scheduled-discrete-time scheme in that the photo detector signals are summed in a common bus concurrently and then converted to voltage levels for digitization. We illustrate that our proposed architecture may be profitable. Predominantly, in solicitations, the bandwidth necessities for a time-multiplexed pattern are highly demanding. In arrears to the up-scaling trend of photo detector arrays motivated by requirements of higher image resolution, the area per readout cell has consistently decreased over the last years and with it the maximum size of integration capacitance. The imminent consequence is that for the conventional readout method the integration capacitance saturates for the same levels of optical flux, which becomes a major limiting factor. The readout cell that we present in this work does not suffer from that effect, as the integration of photo charges is performed on a per row basis in a capacitive Tran impedance amplifier.The key elements of the proposed orthogonal encoding architecture are the active readout cell and the Tran impedance amplifier that performs current to voltage conversion. We describe the readout cell and Tran impedance amplifier operation principle, their properties, major challenges and design methodology. A proof of principle test system is implemented using 0.5μm-CMOS technology to demonstrate functionality of the readout architecture. Test results show it to be a solid option for highly-integrated active and passive imaging systems.