What Resistance Ranges Can Be Used with the NI 9235 and 9236?

Updated Mar 22, 2019

Reported In

Hardware

  • NI-9236
  • NI-9235

Issue Details

  • I am using NI 9235 with 120Ω quarter bridge completion, or NI 9236 with 350Ω quarter bridge completion. The quarter bridge strain gage I would like to use isn't exactly 120Ω or 350Ω respectively. What resistance ranges can I use with the 9235 and 9236?
  • Can I use the NI 9236 with a 120Ω strain gage?

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 Resistance120Ω
Minimum Resistance107Ω
  
NI 9236 
Maximum Resistance394Ω
Nominal Resistance350Ω
Minimum Resistance311Ω

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