Based on the following information I got from the data sheet: http://www.ni.com/datasheet/pdf/en/ds-137 the 6525 already has overvoltage protection. What levels of current and voltage do you expect your device is being subjected to?
"The eight isolated digital input channels break ground loops and offer protection from noise and spikes on external signals."
"Safety from High-Voltage Transients Isolation electrically separates high-voltage front-end channels from each other and the low-voltage back end of the USB-6525. Signals are passed between the two sections of the device using optocouplers. By separating the two sections, any voltages within the isolation specifications are prevented from entering the USB section or other channels. Isolation provides protection for the user, data acquisition system, and measurement data."
I also found this forum post that discusses building circuits for DAQ input protection, which you may find useful: http://forums.ni.com/t5/LabVIEW/DAQ-input-protection-overvoltage-amp-overcurrent-question/td-p/14986...
Hope this helps!
Selene
