An ultra wide dynamic range (WDR) CMOS image sensor (CIS) and the details of evaluation are presented. The proposed signal readout technique of extremely short accumulation (ESA) enables the dynamic range of image sensor to be expanded up to 146dB. Including the ESA signals, total of 4 different accumulation time signals are read out in one frame period based on burst readout technique. To achieve the high-speed signal readout required for the multiple exposure signals, column parallel A/D converters are integrated at the upper and lower sides of pixel arrays. The improved 12-bits cyclic ADCs with a built-in correlated double sampling (CDS) circuit has the differential non-linearity (DNL) of ±0.3LSB.
In this article, we propose a method to extend the dynamic range of the CMOS image sensor to both high illumination and low illumination. A one-frame period is divided into a period of long accumulation for low illumination, and periods of short accumulation for high illumination. The sensor data accumulated for short time are read out several times, and all of data are added repeatedly in a processing unit integrated. The SNR dip at boundary of low illumination and high
illumination regions is reduced by the integration of the short accumulated signals. Because the proposed method is independent of the
type of pixel structure, the low-noise characteristics can be achieved, allowing to use, for example, 4-transistor APS with a pinned photo diode. If the ratio of the long accumulation time and the short accumulation time is 128 and the number of addition times is 8, the dynamic range is extended to 41.6dB and the SNR dip is -11 dB.