There are two problems with your VI. First, the basic logic of writing/appending to a file (Excel, TDMS, anything) should go something like this: Open the file, position yourself at the end of the file, then, in the loop, acquire data and write it to the file until you are finished with data acquisition. When you exit the acquire/write to file loop, then close the file. In particular, the opening and the closing of the file should not be inside the loop.
As others have pointed out, writing to Excel might not be optimal, particularly if you are acquiring data at a high rate (and would therefore be writing a lot of data). We actually use Excel in our data acquisition routine, mainly reading from a WorkSheet to get the parameters of a particular stimulus, but also writing the outcome of the response to the stimulus. As it happens, our "acquisition rate" in this example is on the order of several samples per minute, so there's no problem using Excel (we also sample 16 channels of analog data at 1 KHz -- this gets written as a binary file).
Second, if you really do want to use Excel, use the (existing) Excel file to which you want to append as the "template" argument of the New Report function. Then use the Excel Get Last Row function to position yourself at "end of file", as noted above.
Good Luck.
Bob Schor
