Measurements from a bridge-based sensor are based on a ratio of measured voltage to excitation voltage. NI-DAQmx uses the following equation to calculate that ratio:
where VR is the voltage ratio; V is the voltage output from the bridge; VIB is the initial bridge voltage, as determined by offset nulling; VEX is the excitation voltage supplied to the bridge; and G is the gain adjustment from shunt calibration.
Ratiometric devices divide the voltage output from the bridge by the excitation voltage in hardware. Therefore, V/VEX must be within the device range of the device. On voltage devices, the voltage output from the bridge must be within the device range. The initial bridge voltage and gain adjustment affect the association between device range and input limits. For example, on a device that can measure ±5 V, an initial bridge voltage of 1 V means that the minimum and maximum input limits must correspond to -6 V to 4 V.
NI-DAQmx uses various methods to scale that voltage ratio to physical units, depending on the bridge configuration for strain measurements or on the scaling type that best matches the specifications provided by the sensor manufacturer for other bridge-based sensors NI-DAQmx supports.