How can I adjust the threshold voltage of edge counting?

Hi all,

I'm using PCI-6602 with BNC-2121 for photon counting purpose.

I connect the input to PFI39 channel, nothing is connected to BNC-2121 terminal without this.

After some test, it is clear that my 6602 only count the event which has higher voltage value than 800mV. It has no response(counting) with input signal below 800mV. It only get the count when input voltage increases over 800mV.

But you know, voltage from photomultiplier is usually lower than 800mV. It is impossible to make PMT voltage higher than 800mV for all of my samples.

So I need to find out the way to adjust this kind of 'threshold voltage for counting'.

Thank you for reading.