CMOS Sensors : How CMOS Sensors work

CMOS SENSORS

A typical CMOS is an integrated circuit with an array of pixel sensors. In contrast to CCD, each pixel sensor in CMOS sensors contains its own light sensor, an amplifier and a pixel select switch. An analog-to-digital converter and other components critical to the operation of the pixel sensors are located on the CMOS sensor.

The CMOS sensor contains four main parts: the color filters, the pixel array, the digital controller, and the analog to digital convertor.

• Color Filter

Color filter is the same as was described in CCD based imager.

• Pixel Array

As in the case of CCD, function of the pixel array is to capture the intensity of the light passing through. Each pixel sensor converts the sensitivity of the incoming light to the voltage signal which is then fed to ADC for further processing

There are two types of architectures of Pixel sensors: Passive Pixel Sensor (PPS) & Active Pixel Sensors (APS).

In Passive Pixel sensors, only one photo-detector (without any local amplifier) per pixel is used, whereas in Active Pixel sensors, 3-4 transistors per pixel are used.

Passive Pixel sensors have smaller pixels and large fill factor but they are slow and have low SNR. On the other hand, active pixel sensors are fast, have good SNRs but larger pixels and low fill factor.

However, due to advancement of CMOS technology down to nm, pixel size/fill factor is no longer a big issue and APS is the technology which is preferred and used in most devices.

• ADC

The ADC takes the analog voltage signals from the pixel sensor array and converts them into a digital signal.

• Digital Controller

The digital controller governs the functioning of the the CMOS sensor; it controls the pixel array, ensures synchronism between all pixels, etc.