For my scientific work I wish to register signals coming from a photon detector which generates 50 Ohm TTL signals:
The expected count rate will never exceed 4 MHz.
The devices that need to record these signals are an NI-6343 of which the input impedance is orders of magnitude higher than 50 Ohm and a specialized time tagging device with 50 Ohm impedance on its input.
The time tagging device (TCSPC) and the detector are designed to be used together so there is no issue there. However, for reasons not relevant here, I also need the NI device to register the TTLs.
Everything is connected using Mil spec RG58 coaxial cable and I use a good quality 50 Ohm splitter to run the TTLs towards the NI device and my TCSPC.
Naively connecting things won't work: obviously the input impedance of the NI device is such that no current will flow to it and it 'sees' nothing as long as the 50 Ohm TCSPC is also connected on the other terminal of the splitter.
To address this I think I will use an OPA 2350 of which we happen to have a few in the lab. This opamp is 38 MHz so it might work just fine.
I think I need a unity gain buffer:
However, since my knowledge of electronics doesn't go much further than Ohm's law I am not sure I am not missing something obvious:
- Will the fact that the output impedance of the opamp is never really 0 mess with the voltage that is 'seen' by the NI-6343 as it is effectively in series with the NI-6343? Should I be wary of any other voltage division effects that might occur, effectively lowering the voltage of the output below TTL spec?
- I do not have a -5V supply voltage in the NI-6343, only +5V. Therefore, I would ideally put the supply of the opamp between 0 and +5 but people tell me a 'symmetrical' supply might be better.
Any feedback would be much appreciated!