Hi ASU,
Is sounds like what you are interested in is analog level triggering. Depending on the DAQ card you purchase and use since not all DAQ cards have this capability, you can define a voltage level above which you trigger data acquisition. Everything below that voltage level is not recorded. The Developer Zone article <a href="http://zone.ni.com/devzone/cda/tut/p/id/3017" target="_blank">Analog Trigger </a>discusses what is entailed in analog triggering and how it works. The Knowledge Base <a href="http://digital.ni.com/public.nsf/allkb/5B247F675429568286256F0E007C21E4?OpenDocument" target="_blank">What is the Minimum Signal Width for an Analog Trigger on an NI Data Acquisition Board?</a> discusses the minimum width of a pulse that can trigger the analog reading.
Hope this explanation helps.
Mallori M.