Hello,
I want to use the PXI-7811R with the SCH68-68 cable in order to control some custom circuitry. I will use the 7811R for now as an output only. The signal integrity and timing are important so I would like to determine the best type of termination to use in the custom circuitry in order to minimise relfections. I could not find any published specification for the characteristic impedance of the SHC68-68 cable (I found some information on some of the other 68-pin cables, however not specifically the SHC68-68), additionally there was nothing about the output impedance of the 7811R, except for an indirect mention in the following KnowledgeBase article: http://digital.ni.com/public.nsf/allkb/D58C269F7771770E86256F490082E6C8
"The output impedance for the R Series devices is non-linear due to the circuitry used to provide 5V tolerance. The output impedance is low (<50 ohms) for output voltages near 0V. As the output voltage increases to around 3.3V, the impedance rises to approximately 100 ohms. As the voltage rises beyond 3.3V, the impedance rises very quickly to megohms."
I would like to find out what the characteristic impedance of he SHC68-68 is, and to verify that the above statement in the KnowledgeBase definitely applies to the 7811R, and if so then whether more precise information is available - i.e. how much exactly is meant by "<50 ohms", as this in combination with the cable's characteristic impedance will directly relate to how much signal is reflected for a second run towards the receiver in case the receiver is not properly matched.
Of course I could determine these answers by careful experimentation, however I was quite surprised that this information was not published so I wanted to see if these figures are at all available before spending time determining them myself.
Many thanks in advance!