Beginning around 2 years ago we began to have sporadic reports of intermittent video that we call “flicker” from one or more of the 8 input ports on a video switcher. Each input port has an ADV7623 behind it. There are also 3 auxiliary HDMI inputs (for video conferencing systems and such) and 4 HDMI outputs for up to 4 monitors. 8+3+4 is a total of 15 ADV7623’s per system. Between the 11 ADV7623's on the inputs and the 4 ADV7623's on the outputs are 4 ADN4604 cross-point switches. The ADN4604 routes one of the 11 inputs to any of the 4 HDMI outputs.
Most recently the problem has gone from sporadic to frequent. Based on input from ADI the PCB was spun to add more decoupling capacitance. This made no difference. I honestly don't think this problem is due to ineffective decoupling of the power supplies.
Cooling the ADV7623 associated with an input that is "flickering" stops the flickering. Heating the ADV7623 associated with an input that is flickering makes the problem worse.
We swapped ADV7623's between input ports and the "flickering" follows the ADV7623 not the input port. Based on this, I don't think the problem is layout related.
We've seen flickering on all 11 of the inputs, but Port 1 has a slightly higher reported incidence. We believe this is due to the fact that our customers use Port 1 most often. This supports the notion that the problem is not layout related.
We have found through experimentation that the incidence of “flicker” can be reduced by changing the value of EXT_SWING from the 887 ohms ADI recommends to 750 ohms. I personally don’t see how EXT_SWING can affect the input to the ADV7623, only the output differential swing. They have also swapped ADV7623’s between channels and found that the problem follows the ADV7623. Certain production lots of the ADV7623 are worse than others.
To be clear, I'd like to know the value of Cpg1 and Cpg2 as defined in AN-1260. AN-1260 uses a rule-of-thumb of 5pF each. I'm looking for a guaranteed specification for these two values.