You are only exporting the very last iteration of the while loop. Right click on the exit tunnel and select ' Enable Indexing'. This will create a 2D array and you will have to change your wiring. Or, place the Write to Spreadsheet function inside the loop. With it outside, the array allocation will cause the VI to run slower and slower the longer you run the loop. If you have it inside, you will slow down each iteration by a fixed amount of time - the time it takes to write all of the samples with each run. You should look at the consumer/producer design pattern.
The niDMM Read can also return a waveform data type. If you need the timing information (t0 and dt), you should be using this mode. The Export Waveforms to Spreadsheet File can be used for this.
