Actual Excitation Voltage of NI 9237 Differs from Specified Voltage

Issue Details

• When creating a DAQmx Task for Strain or Custom Voltage with Excitation with the , the NI9237 excitation voltage that I specified in the task is not the same as what I measure across Ex+ and Ex-. For example, when I specify an excitation voltage of 6 V,  the actual voltage being output across Ex+ and Ex- is only 5 V. Why does the actual voltage differ from the voltage I specify?

• Can I use a module like the NI 9237 to provide the 10Vdc  and 28mA signal excitation that my torque sensor need?

Solution

When using more than one channel on the NI 9237 and selecting an excitation voltage, you need to be aware that the module can provide up to 150 mW of excitation power. The NI 9237 automatically reduces internal excitation voltages as needed to stay below 150 mW total power, unless you supply an external excitation voltage at the module's 4-pin EX+ and EX- connector.

All measurements taken by the NI 9237 ADC are ratiometric which means they are a ratio of the voltage output by the bridge and the excitation voltage. This mitigates the effect of small drifts in excitation voltage. 

Refer to the related links for more information about when and how to use external excitation with an NI 9237.

Additional Information

The 150 mW limit allows you to power half and full bridges as follows:

• Four 350 Ω half bridges at 5.0 V
• Four 350 Ω full bridges at 3.3 V
• Four 120 Ω half bridges at 2.5 V

For more information, see the Excitation Voltages section of the NI 9237 Datasheet .