A sample sensor embedded with the readout circuits shows double the performance of a conventional one.(1) Toshiba presented this development at ISSCC 2013 in San Francisco, CA on Feb. 20.

As demand for commodity mobile phones takes off in emerging markets, CMOS image sensor need to be smaller, consume less power and offer low noise performance. The pixel readout circuits of CMOS image sensors are largely noise reducing correlated double sampling (CDS) circuits, along with a programmable gain amplifier (PGA) and an analog to digital converter (ADC). Serial signal processing architecture is best suited for securing conventional CMOS image sensors with a small area and low power pixel readout circuits, because a PGA and ADC can be shared by many CDS circuits placed in each column area of the sensor. However, smaller size and lower power are still challenges, since noise reduction circuits occupy a large area in the readout circuits, and PGA and ADC have high power consumption.

Toshiba has developed three key technologies to overcome these challenges:
1) Column CDS circuits primarily made up of aria-efficient PMOS capacitors. The area of the CDS circuits is reduced to about half that of conventional circuits.
2) In the readout circuits, a level shift function is simultaneously achieved by a capacitive coupling through the PMOS capacitors, allowing adjustment of the signal dynamic range between the column CDS circuits and the PGA and the ADC. This achieves low power and low voltage implementation of the PGA and ADC, reducing their power consumption by 40%.
3) Implementation of a low power switching procedure in the ADC suited to processing the pixel signals of CMOS image sensors. This reduces the switching power consumption of the ADC by 80%.
Toshiba has integrated the three technologies in a sample sensor and confirmed that they double the overall performance of the sensor core. The company now plans to apply CMOS image sensors with the readout circuits to low cost mobile phones and medical cameras in fiscal year 2013.

(1)    Compared using Figure of Merit; FOM = power x noise / (pixels x frame rate)