Use of External Excitation With the NI 9237 Module

Issue Details

• What is the correct way to connect the external excitation to an NI 9237?
• What is the purpose of the NI 9942 accessory?

Solution

There are two versions of the NI 9237, one has a Dsub connector and the other one has RJ-50 connectors. The connection of the external excitation will depend on these variants, you can find the pinout of both connectors on page 10 of the NI 9237 Getting Started Guide. The excitation pins for each channel are also listed 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

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

Additional Information

• The 9237 can provide 10 volts of excitation. If an external power supply must be used, do not exceed 10 V because it must be connected through the NI 9237. Exceeding this voltage can damage the module.
• For a full bridge measurement, the NI 9237 is only measuring differential voltage and is 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.
• 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.
• If you are using Internal Excitation, do not connect an external power supply. The external excitation terminals are directly connected to the Ex+ and Ex- of each channel, so the sensor will be receiving power from both internal and external power sources, and that can have different consequences in the module or the measurements. 
• 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.