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Linear Voltage/Current Dual Mode Image Sensor

We have developed a CMOS image sensor capable of both voltage- and current-mode operations. Each pixel on the imager has a single transistor acting as either source follower for voltage readout, or transconductor for current readout. The two modes share the same readout line, but have their own correlated double sampling (CDS) units for noise suppression. The 300x200 image array is a mixture of 3 different types of pixels with identical photodiodes and access switches; while the readout transistors were optimized for their designated mode of operation. This ensures a fair comparison on the performance of the different modes.

Fig. 1 shows one pixel in the imaging array with its readout circuitry. The pixel itself is based on the classical 3T APS design. It includes a photodiode, a readout transistor M1, a reset switch M2 and a column selection switch M3. An additional transistor, M4, is used because the pixels are reset individually. It implements an AND function such that the reset pulse (which is common to a row) only triggers the pixel
in the selected column. Likewise, a row selection switch exists at the end of every row of pixels. The row switches, together with the column switches in the pixels, enable the random access of any pixel in the array.

In voltage-mode readout, the mode selection switches are set to position 1. This connects the drain terminals of all the readout transistors to VDD. The source terminal, on the other hand, is connected to a constant current bias I1 (only for the selected pixel). Accumulated photocharge is detected using the readout transistor as a source follower, which helps to drives the readout line with a large capacitance. The output circuitry include a voltage-mode CDS for FPN suppression, and a buffer to drive the output pad.

Current-mode operation corresponds to position 2 on the mode selection switches. In this mode, the common terminals of the readout transistors are connected to ground, while the readout line connects to a current conveyor which masks its large capacitance [3]. Under specific bias conditions, the readout transistor in the selected pixel operates as a transconductor. It converts the photocharge linearly to its drain current, which is then copied by the current conveyor and processed by the CDS. Note that the drain and source of M1 have exchanged roles in this mode, and readout current now flows into the pixel array. The current source I1 merely adds a constant offset current to meet the input range of the CDS.

References:
[1] Z. Yang, V. Gruev and J. Van der Spiegel, "A CMOS Linear Voltage/Current Dual-Mode Imager," Proc. IEEE ISCAS, Kos, Greece, May 2006. (Upenn, IEEExplore)