NI 9237 Internal Resistor Nominal Resistance Specifications

Updated Mar 20, 2019

Reported In

Hardware

  • NI-9237

Issue Details

I was looking through the User Guide for the NI 9237 and saw no nominal resistance specification or analog input impedance for the internal resistor like I might expect. What is the resistance value, and why is this not specified?

Solution

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:
 
  • Full Bridge:
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.
 
  • Half Bridge:
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. 
 
  • Quarter Bridge:
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.

Additional Information

Non-Standard Strain Gages

When using non-standard strain gages, use one of the three configurations above with external precision resistors to create a balanced bridge. For a strain bridge to be balanced, both legs do not need to be equal. However, the resistors on each leg must be the same where R1 = R2 and R3 = R4. To  use a non-standard strain gage, use the NI-9237 as directed in "Quarter Bridge" configuration above with a matching completion resistor.

For example, if your strain gage is 155 Ohm, you would need an extra 155 Ohm precision resistor to complete the external leg (see "Rexternal" in Figure 1). Using these in tandem will create a balanced bridge.

Figure 1: Configuring the NI-9237 for Quarter Bridge measurements

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