Measure a video ADC's differential gain and phase

February 15, 2016 // By Sambhav Jain and Abhijan Chakravarty
Measure a video ADC's differential gain and phase
Sambhav Jain & Abhijan Chakravarty consider how to measure a video ADC's differential gain and phase.

A composite video signal consists of luminance and chrominance, both of which directly affect the quality of a video that you might see. Because most video today is digital, a video ADC is necessary to digitize the composite video signal. From the digital output, you can calculate the ADC’s differential phase and differential gain.

Luminance contains the information about brightness while chrominance contains the information about the color. Chrominance, in turn, has two components: saturation and hue. Saturation (intensity of the color) is determined by the amplitude of a video signal's sub-carrier, while hue (shade of the color) is determined by the phase of the sub-carrier. The quality of video depends on how accurately a video ADC converts the analog video signal. The path from video source to the display is long and the system should make sure that any error in converting the video signal should not become large enough to be perceived by the viewer.

There are two important specifications of a video ADC: differential gain and differential phase, both of which affect luminance and chrominance. There are several methodologies to measure these parameters, but to measure them accurately on the bench is a challenging task.

We use a technique called staircase methodology to measure these parameters. We'll describe how that works while explaining some practical limitations to the method.

Differential Gain & Phase
Differential Gain can be defined as the error in the amplitude of chrominance signal due to change in luminance level. Think of it as change in amplitude of sinusoidal
signal by changing DC offset of input signal. This error varies color saturation with brightness. For example, a red rose turns pink as evening approaches.

Differential phase can be defined as the error in the phase of chrominance signal due to change in luminance level. Here, the phase of the sinusoidal signal changes
with DC offset. This error varies the hue or the shade of

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