A low-photocuffent CMOS retinal focal-plane sensor with a pseudo-BJT smoothing network and an adaptive current Schmitt trigger for scanner applications

Abstract

In this work, a new structure of low-photocurrent CMOS retinal focal-plane sensor with pseudo-BJT smoothing network and adaptive current Schmitt trigger is proposed. The proposed structure is very simple and compact. This new circuit can easily be implemented in CMOS technology with a small chip area. Another innovation of this circuit is that the proposed circuit could be operated for low-induced current levels (pA), and the current hysteresis of the proposed current Schmitt trigger could be adjusted adaptively according to the value of induced photocurrents. In this work, the detection of static and moving objects, such as a moving white bar, are proven by projecting a pattern through HSPICE simulation. The proposed retinal focal-plane sensor includes a 32 x 32 pixel array with a pixel size of 70 x 70 mum(2). The fill factor is 75% and the total chip area is 3000 x 3030 mum(2). It is with fully functional 32 x 32 implementations consuming less than 8.8 muW per pixel at 3.3 V. Measurement results show that the proposed new retinal focal-plane sensor has successfully been used in character recognition of scanner systems, such as pen scanners, etc.