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Counting Short TTL Pulses with a Digilent DAQ



We are looking at purchasing a Digilent DAQ for the following application, but are confused as to the specifications required and thus the model we should purchase. We have a single-photon detector that is an avalanche photodiode (APD). When this detector detects a photon (i.e. an event), it produces and outputs a TTL pulse that is typically 20 ns in width with < 2 ns rise and fall times. It is not important for us to resolve the time that the TTL pulse arrives at the counter on the DAQ, however, it is important that we do not miss counts. I have noticed that the USB-1208HS series features counters that have a maximum input frequency of 20 MHz. My understanding is that this corresponds to bin width of 50 ns. Please correct me if I am misguided, but does this mean that this module could potentially record multiple TTL pulses in the same bin, leading to a count of 1 that should be 2? If so, are there any other products that would be more suitable?


For some additional information, here are some more desirable specifications for the DAQ we are hoping to purchase:

  • Near 1 MHz sampling rate
  • Analog I/O & Digital I/O with at least two dedicated counters
  • Support for Python control
  • Connected to computer via USB


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The USB-1208HS counter input minimum pulse width is 25nS, and the input bandwidth is 20 MHz.  Our highest-performing counter inputs can be found on the USB-1808X. They are rated at 50 MHz. However, it might not meet your requirements for analog input speed and the number of digital IOs. One nice feature is that it can read analog, digital, and counter inputs simultaneously. 



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