Actual Excitation Voltage of NI 9237 Differs from Specified Voltage

I am using an NI-9237 module to perform measurements with a bridge-based sensor. When creating a DAQmx task for Strain or Custom Voltage with Excitation, I configure an excitation voltage; however, the measured excitation voltage across the Ex+ and Ex− terminals does not match the specified value.

For example, when configuring an excitation voltage of 6 V, the measured output across Ex+ and Ex− is approximately 5 V. This difference affects the expected measurement accuracy.

Additionally, I need to determine whether the NI-9237 can be used to provide excitation for a sensor that requires 10 V DC and 28 mA.

Symptoms include:

• Measured excitation voltage differs from the configured value
• Lower voltage observed across Ex+ and Ex− terminals
• Uncertainty about whether the module can meet higher excitation requirements (10 V, 28 mA)

The difference between the configured and measured excitation voltage is expected behavior and is related to the supported excitation levels and total power limitations of the NI 9237 module.

The NI-9237 supports discrete excitation voltage levels of 2.5 V, 3.3 V, 5 V, and 10 V. If a value outside of these levels is configured, the module adjusts the excitation voltage to the nearest supported value.

In addition, the NI-9237 limits the total excitation power to 150 mW. If the configured excitation voltage and connected load require more power than this limit, the module reduces the excitation voltage automatically.

Check the bridge resistance and configured excitation voltage to ensure that the required power does not exceed the module limits.

Example:

Consider a bridge sensor with a resistance of 350 Ω and a configured excitation voltage of 10 V. The required power can be calculated as:

P = V² / R
P = 10² / 350
P = 100 / 350
P ≈ 0.286 W (286 mW)

This exceeds the NI-9237 limit of 150 mW, so the module cannot maintain the configured 10 V.

To determine the maximum allowable excitation voltage within the power limit:

V = √(P × R)
V = √(0.15 × 350)
V = √52.5
V ≈ 7.24 V

The excitation voltage is therefore reduced automatically to stay within the allowable power range.

When using multiple channels, the total excitation power is shared, which can further reduce the available excitation voltage per channel.

The NI-9237 performs ratiometric measurements, meaning the measurement is proportional to the excitation voltage. This reduces the effect of excitation voltage variations on measurement accuracy.

If your application requires higher excitation power (for example, 10 V at 28 mA), use an external excitation source connected to the Ex+ and Ex− terminals.