CMOS 32-APS linear array for high-sensitivity, low-resolution spectrophotometry

This paper presents a CMOS 32-APS (active pixel sensor) linear array for high-sensitivity, low-resolution micro spectrophotometer. The APS employs a buried double junction (BDJ) detector, which has an improved spectral response when compared to the use of a CMOS photodiode, and allows wavelength discrimination for instrumental auto-calibration. The sensitivity is enhanced by using large pixel and small extra integration capacitor, with implementation of high-gain charge amplifier to minimize effects of detector's junction capacitors. The APS features two channels: well channel and diffusion one. Each channel performs charge amplification and correlated double sampling (CDS) with buffered output. The well channel is read for photometric measurement, while both channels are exploited for wavelength detection. The in-pixel CDS circuits allow synchronous sampling of all APS, which improves readout rate. The fabricated linear array in a 0.8-μm CMOS process has been tested. The evaluated sensitivity of the APS is about 14 V/lx s @ 555 nm for the well channel. Under a 3.3 V supply, the dynamic range (DR) of the APS for a fixed integration time is about 50 dB for both channels. By varying integration duration, a DR over 130 dB for both channels can be reached. For photometric measurement, the minimum detectable signal using 125 s integration is about 1 μlx @ 555 nm.