Electronic – Noise at 2^n transitions

adcimage-sensornoise

I am sampling a video signal from a linear imager. I noticed when the illumination is just at the intensity when the ADC increases by another MSB bit at the output, the noise occurs. That is, when ADC transitions from 0111 to 1000, 01111 to 10000 and so on. It happens at every transition. At least, this is how it appears to me. What could be the source of such noise?

enter image description here

I suspect, this happens somewhere on the digital side of the circuit. Here's the quantitative profile of the image intensity at the line20:

enter image description here

Here's the sample clock signal before and after the gate buffer:

yellow: 0-3.3V

blue:0-5V

enter image description here

EDIT:
In short: the problem was with timing that is discussed in this post and the bad grounding that is discussed here

Best Answer

Placeholder's valid comments notwithstanding, the noise you are seeing is indicative of a timing skew problem among the digital bits coming out of the ADC. Instead of xxx10000 and xxx01111, you are occasionally getting xxx00000 and xxx11111.

I see the clock going into the ADC, but I don't see the next place in the datapath where the data is clocked again. Be sure that the timing delays (resistors, buffer, connectors, wiring, etc.) on the data lines still meet the setup and hold requirements for wherever that reclocking occurs. Pay attention to clock skew among the boards, too.

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