How Do I Use External Excitation with the NI 9237 Module?

Issue Details

I am trying to find out if there is a possibility to use the 9237 module to measure a load cell with a 700 ohms full bridge using external excitation. As the manual is does not specify details, is there any documentation on how to complete the connections?

Solution

Yes, using a load cell of 700 Ohm is not a problem. For a full bridge measurement, the 9237 is only measuring differential voltage and is thereby independent of the nominal resistance of the strain gauge. The input impedance of the NI 9237 is essentially an open circuit and should not affect the bridge based measurement.
 

As for the external excitation voltage, there are two versions of the 9237, one with a Dsub connector and one with RJ-50 connectors. You can find the pinout of both connector variants of the device in the NI 9237 Getting Started Guide and the excitation pins for each channel are also noted below:

Dsub

• Ch 0: EX0+/EX0- on pins 2/21
• Ch 1: EX1+/EX1- on pins 6/25
• Ch 2: EX2+/EX2- on pins 12/31
• Ch 3: EX3+/EX3- on pins 16/35
• Excitation Voltage Source: V_EX+/V_EX- on pins 10/29

RJ-50

• Ch 0: EX+/EX- on pins 6/7 on CH0
• Ch 1: EX+/EX- on pins 6/7 on CH1
• Ch 2: EX+/EX- on pins 6/7 on CH2
• Ch 3: EX+/EX- on pins 6/7 on CH3
• Excitation Voltage Source: EX+/EX- on NI 9942 connector. The NI 9942 connector is not included with the NI 9237 and must be purchased separately.

Additional Information

The 9237 can provide 10 volts of excitation. If an external power supply must be used, because it must be connected through the NI 9237, do not exceed 10 V even when using an external power supply. Failure to comply with this can damage the module.

When you enable external excitation, the excitation ± terminals (e.g. pins 6 and 7 on the RJ-50 version) disconnect from the internal cDAQ backplane source (used for internal excitation) and connect to the excitation voltage source pins on the front of the device. With external excitation enabled, a 2-10 volt external power supply needs to be connected to those pins in order to provide excitation. The externally supplied voltage is passed directly through to each channel and is not affected by setting a different excitation value in software.

However, if external excitation is selected but an external excitation source is not connected, there will be a voltage present on the EX ± because these channels are not completely decoupled from the internal excitation source. The voltage here is relative to any previous external excitation source that may have been connected.

When purchasing an external power supply for use with the 9237, the accuracy and precision of the power supply are not particularly important because the 9237 uses ratiometric measurements, meaning that most of these inaccuracies will be accounted for by the device itself.