AD Converter

AD Converter
By Vincent Bockaert

Sensors consist of pixels with photodiodes which convert the energy of the incoming photons into an electrical charge. That electrical charge is converted to a voltage which is amplified to a level at which it can be processed further by the Analog to Digital Converter (ADC). The ADC classifies ("samples") the analog voltages of the pixels into a number of discrete levels of brightness and assigns each level a binary label consisting of zeros and ones. A "one bit" ADC would classify the pixel values as either black (0) or white (1). A "two bit" ADC would categorize them into four (2^2) groups: black (00), white (11), and two levels in between (01 and 10). Most consumer digital cameras use 8 bit ADCs, allowing up to 256 (2^8) distinct values for the brightness of a single pixel.

This 8 bit Analog to Digital Converter (ADC) "samples" the analog voltages into 256 discrete levels which are assigned a binary label consisting of zeros and one

Digital SLR cameras have sensors with a higher dynamic range which are able to capture subtle tonal gradations in the shadow, midtone, and highlight areas of the scene. Because those sensors output pixel voltages with very minute voltage differences they are usually equipped with 10 or 12 bit ADCs which allow for more precise categorization into 1,024 or 4,096 discrete levels respectively. Normally such cameras offer the option to save the 10 or 12 bits of data per pixel in RAW because JPEG only allows 8 bits of data per channel.

Often, marketing material advertises the bitrate of the ADC to suggest the digital camera or
scanner is able to output images with a high dynamic range. From the above it is easy to
understand that this is only true IF the sensor itself has sufficient dynamic range. As shown in the table below, if the sensor has a low dynamic range to begin with, using an ADC with a higher bit rate will only help to increase sales to uninformed buyers.

Sensor Dynamic Range	ADC Type	Image Tonal Range
Low (e.g. around 256 tonal values)	8 bit	8 bit
Low (e.g. around 256 tonal values)	10 or 12 bit	8 bit
High (e.g. around 4,096 tonal values)	8 bit	8 bit
High (e.g. around 4,096 tonal values)	10 or 12 bit	10 or 12 bit if RAW