Encountered issue when counting number of APD pulses using counter channel

[amatuerquantumengineer]

My project has been using an NI6229 board's counter channel to get the photon counts by counting how many APD pulses we get. Recently, we want to use the same signals to implement some other functionalities, so I branched out the APD pulse using a T-connection. Because of that, the signal peak going into the NI channel went from 3.3V to around 2.4V as seen on a generic scope (corresponding to a root 2 power splitting, which makes sense). Pulses attached below. The NI counter channel now doesn't record anything. What could be possible causes of this?

One possibility we thought of is that the trigger threshold for a pulse is too low now, but the specs say logical high voltage is from 2.2V to 5.3V, and our signal is safely within that range. Where can we read more about how each pulse is counted?

Thanks a lot in advance.

[Kevin_Price]

I think you need a simple interface chip that will turn your rather analog-looking pulse into a TTL-compatible digital pulse.  Exactly *which* chip isn't my area of expertise, but a Schmitt trigger could be one option.   Maybe an EE around here will have a better specific recommendation.

-Kevin P