Need Help with design issue

[Roff]

Why have you used 43Ω instead of 51Ω?

Because the LCX244 has finite output resistance. I estimated it to be about 30 - 40 ohms per driver. Five in parallel gives 6 - 8 ohms total. 43 ohms in series gives 49 - 51 ohms total.
It seems our OP does not understand the importance of trying to maintain a 50 ohm output resistance. Below are plots of 50 and 100 ohm sources into ≈30 feet of coax into a 50pF load. It looks pretty much the same with no load. Notice that the first pedestal is very close to the logic '1' threshold, which can cause noise to occur on the output of a receiver.
Tom, 50 ohms output resistance is not an arbitrary number. For best pulse fidelity, the source impedance needs to match the characteristic impedance (Z0) of coax. Most test coax has Z0=50 ohms.

 

[tommurphy73]

Hello Ron,

Thanks for that explanation. I had not considered the output impedance of the lcx244 in my calculations and had presumed that the only impedance was the external resistor. I will use the resistor value as you have indicated and compare the traces on the scope afterwards. Unfortunately I have no way of generating an image from my scope so that I can post the results.

Regards
Tom

 

[Roff]

Hello Ron,

Thanks for that explanation. I had not considered the output impedance of the lcx244 in my calculations and had presumed that the only impedance was the external resistor. I will use the resistor value as you have indicated and compare the traces on the scope afterwards. Unfortunately I have no way of generating an image from my scope so that I can post the results.

Regards
Tom

How many gates will you be using in parallel? With 5 in parallel, I don't believe 43 ohms from the output,shorted to ground, will exceed the spec in the datasheet.

 

[tommurphy73]

I will be using 6 gates in parallel.

Tom