The NI 9237 returns a ratiometric value which means the data you get back is in mV/V, a unitless measurement. The device does the division in hardware which allows for a more accurate reading since it uses the actual excitation voltage in the calculation. This makes the device ideal for bridge and strain measurements, but also means that you cannot not connect the excitation terminals and attempt to read a voltage. In DAQmx, users are restricted to two types of channels for the 9237: AI Strain Gage and AI Custom Voltage With Excitation.