The NI 9237’s internal resistors for half bridge completion are 10 kΩ each. High input impedance of analog channels should not have an effect on strain measurements.
When working with the NI 9237, there are three possible configurations:
When measuring a full bridge sensor, all four legs of the Wheatstone bridge are external to the NI 9237. While the nominal resistance of the strain gage (Typically 350 or 120 Ohms) will need to be configured in software so that the measurement is scaled properly, the NI 9237 is simply measuring differential (ratiometric) voltage, and therefore no resistance specification is necessary.
When measuring a half bridge sensor, the NI 9237 can be used to complete the Wheatstone bridge by internally supplying two of the four necessary resistors. Because these two resistors constitute one leg of the bridge, they do not need to match the nominal resistance of the half bridge strain gage. They simply need to be of matched resistance themselves so the excitation voltage drops equally across them. Therefore, the nominal resistance does not need to be specified.
When measuring from a quarter bridge sensor, the NI 9237 must be used in half bridge completion mode and an external Quarter Bridge Completion Accessory must be used. The resistor of this completion accessory must match the nominal resistance of the quarter bridge strain gage. For this reason, the nominal resistance of these accessories is specified. For example, the nominal resistance of the resistor internal to the NI 9944 Completion Accessory is 120 Ohms, while that of the NI 9945 Completion Accessory is 350 Ohms.