Solution
The NI 9235 and 9236 perform quarter bridge completion of a Wheatstone bridge. The 9235/9236 specifications linked below state that the full range voltage for these devices are ± 29.4 mV/Vexc. We can leverage that range along with the Wheatstone bridge formulas shown below to figure out the ranges of our devices.
Figure 1. Wheatstone bridge circuit
Figure 2. Wheatstone bridge formula
Figure 3. Simplified Wheatstone bridge formula for quarter bridge completion
In the formulas above Vr/Vexc is the ± 29.4 mV/Vex specified in the manual. R1, R2, and R3 will all be 120Ω for the NI 9235, and 350Ω for the NI 9236. Using these formulas, you can solve for Rx to be the values shown in the table below.
Table 1. NI 9235 and NI 9236 Resistance Ranges| NI 9235 | |
|
Maximum Resistance
| 135Ω |
| Nominal Resistance | 120Ω |
| Minimum Resistance | 107Ω |
| | |
| NI 9236 | |
| Maximum Resistance | 394Ω |
| Nominal Resistance | 350Ω |
| Minimum Resistance | 311Ω |