Look at the manual of the NI 9215 http://www.ni.com/pdf/manuals/373779f.pdf.
In particular you have to check if you are measuring a floating differential signals with or without ground. In the second case you have to garantee the use of a 1M resistor as it is shown in page 13.
thanks for prompt your reply. Infact I didn't connect the resistor to the COM. Since I don't have the resistor, I have connected the sensor to the NI9234 and it doesn't work with the NI9234 niether. Do you have any suggestion?
For this purpouse you have to check the connection made in page 8 of http://www.ni.com/pdf/manuals/374238c.pdf.
But in this case you are using BNC, right?
In any case you have to check the floating connection.
I used the BNC connector as pointed out in the manual.
What do you mean I have to check the floating connection?
Ok, you are using BNC connection. This sensor is compatible with NI9234 and NI9215.
If you are using NI9215, make sure the connection with COM is made by pull down resistor, instead if you are using NI9234 with BNC there are no problems.
As I wrote last days, you have to check the correct task that you have create and make sure you sensor still works fine.
Let me know.