convert counter rate to DC analog

hi,all

    I use PCI 6601 and NI BNC-2121 to count the pulse from avalanche photodiode( APD,PerkinElmer SPCM AQR 14). The photon intensity detected by APD can be displayed on the labvew software. It works well.  But previous members in my lab did the build-up job. I am new to NI hardware and labivew.

  Now I am trying to feed the TTL signal from APDs （SPCM AQR 14) to an Atomic force microscopy (AFM)  controller. But only DC signal can be fed to the BNC channels  on that controller.  So  I need to not only count  pulses but also convert the TTL  pulses into a DC voltage depending on the pulse rate. The DC voltage can then be fed into the controller.  Is there any solution offered by NI?

  Thanks!