Roff
Well-Known Member
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.Why have you used 43Ω instead of 51Ω?
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.
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