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Calculate video ADC's differential gain, phase

01 Feb 2016  | Sambhav Jain, Abhijan Chakravarty

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A composite video signal is made up 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 digitise 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 colour. Chrominance, in turn, has two components: saturation and hue. Saturation (intensity of the colour) is determined by the amplitude of a video signal's sub-carrier, while hue (shade of the colour) is determined by the phase of the sub-carrier. The quality of video depends on how accurately a video ADC converts the analogue 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 and 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 colour 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 the colour with brightness. Figure 1 shows a representation of these errors.

Figure 1: Illustration of Differential Gain and Differential Phase.

Measurement methodology
There is no single predefined method for measuring differential phase and gain. Several methods can be used such as the Ramped-Sine method, Staircase method, or by overlapping slow sine on fast sinusoidal signal. The intent of each of these methodologies is to check how good a Video ADC can reproduce a fast varying sinusoid with slowly changing DC offset. We use the Staircase method because both differential gain and differential phase can be measured using the same setup in single test.

In the staircase method, the ADC's input is a sinusoidal signal overlapped on each step of a staircase signal that covers the entire ADC range. This signal can be created by simply varying the DC offset of a sinusoidal signal coming from a waveform generator. Figure 2 is a typical video ADC output when there is a sine-overlapped staircase waveform at its input.

Figure 2: The test signal consists of a sine wave overlapped with staircase function.

Measurement granularity and accuracy of the measurement improves as the number of steps increases. The number of steps should be decided in such a way so as to balance between the accuracy of the measurement and the test time. Amplitude of the sinusoid should be around 20% of full input range and staircase should cover entire ADC range. Peak-to-peak amplitude and phase are calculated for each step of the staircase.

There are quite a few methods for calculating the phase and amplitude of the ADC's output signal. You can use frequency domain techniques, but accuracy decreases as the number of samples decreases per step. That's why there is trade-off between test time and accuracy. You can also use a time-domain approach like Quadrature Sampling.

Quadrature sampling technique
Any sinusoidal signal can be split up into two amplitude modulated sinusoids which are phase apart from each other by 90 degrees. These sinusoids are called In-phase and Quadrature components of the signal. By using these two signals, we can synthesise the actual signal. A mathematical representation of these components is shown in the equation below. It shows the in-phase and quadrature components of a sinusoidal waveform.

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