we are still tweaking our board but it is working with WUXGA (154MHz). We finally ended up with 50Ohm routing and 22Ohm resistors. I wounder why you did choose a 75Ohm routing on the PCB?
I have not verified with the original designer of that board however for his design 75 Ohm series resistors worked fine. It's related to the board impedance. As with all designs you may need to tweak the series resistor to match your board impedance. On my designs I generally spec it as 22-33 Ohm series resistors on a 55 Ohm board and assume the output impedance of the chip is about 25 Ohm.
As a general rule the higher the series resistor, the slower the edge slew rate, the lower the possible EMI emissions. For a final product you have to balance EMI emissions against functionality at the maximum clock rate by adjusting series resistance to board impedance and device impedance.
that is all clear to me. I did not get the point why 75 Ohm traces we're used? Sound like this could have been 50Ohm as well.
The 7612 LLC pin has a 75 Ohm output impedance. CN-0224 says to run a 75 Ohm trace with no series resistors and end terminate to a split 75 Ohm load. Which is the absolute best way to run a 75 Ohm source.
For the pixel bus it uses 50 Ohm series with no termination.
I can tell you from experience with the 7612-7511 evaluation board we used 55 Ohm boards/traces with 33 Ohm series resistors for all the lines and do not have issues running a max clock rates.
And why did you choose 75Ohm over 50Ohm? Just because it is video?
I will see if I can get a direct answer from the guy who wrote the circuit note.
Thanks, the reason I'm asking is because our pcb house did some polar simulation and found that 50Ohm is more stable in their process with the stackup that we did choose than 75Ohm. We ended up with 22R serie resistors and 150R resistor on the llc and 50ohm traces on clock and video bus. Did not test 100R yet. Would be worth testing.
The best solution is to run signal integrity checks with the IBIS file and tweak the layout /series resistors. Also the drive strengths are adjustable. I'd start with a 50 Ohm board and check the integrity.
Just an FYI, all the boards I've designed are 50 Ohm boards and I haven't seen any issues with the devices using this digital driver. I'll still try to connect the circuit note writer to see exactly why he did it this way. (He may have been using lower drive strength for his design.)
yes we did and that was consistant with our findings.
Just an update, the original board was designed as a 75 Ohm board so CN-0224 used the right resistors.
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