This behavior is preferable for bridge measurements since the voltage across the bridge is directly proportional to the excitation voltage. If the returned measurement were in terms of volts variations and errors in the excitation voltage would be reflected in the measurement; however by returning a ratio, these variations can be factored out. This allows fixed strain to remain fixed even if the excitation voltage fluctuates.
If this is not behavior you want and would rather directly measure voltage instead of volts per volts of excitation the NI 9215 and NI 9219 are analog input modules that will measure voltage. The product pages for these modules are linked in the related links
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